Hi Brownie19,
I can see that you’ve been doing some “homework” 
You are right about Natura and in some cases this has worked to consumers’ benefit when retailers are looking to clear their old Natura models at reduced prices (some have decided not to continue with Natura because of some of the issues they were having or have decided to take a wait and see approach).
In general they are good quality mattresses but as you mentioned they tend to carry a fairly high price tag and theiqr “value” would depend on the prices charged by the retailer you are dealing with and on any other benefits of the mattress and retailer that are important to you. Knowing ILD (which is a “comfort” spec and not a “quality” spec) is not really important when you are testing mattresses locally because your testing will tell you more about the PPP of a mattress than knowing the ILD of the layers. Of course it can help if you are hoping to use a mattress as a guideline for an online purchase.
As you mentioned … latex is a much different type of material than memory foam and is closer to good quality polyfoam (which is also a fast response material) in it’s “feel”.
so called “plant based foams” are more about greenwashing than reality and they are certainly not “all natural” or even close to it. There are two main chemicals used to make polyfoam with one being an isocyanate and the other being a polyol. Plant based foam have replaced some percentage of one of these chemicals (the polyol) with a plant based alternative but in most cases the plant oil has also been chemically altered to make it suitable for use in making foam. In most cases the replacement percentage is under 20%. As you mentioned it is still polyfoam that uses a little bit more renewable feedstock (and it’s still not biodegradable).
1.8 lbs would be the minimum density I would consider in a higher priced mattress and 2.0 lbs or better would be better yet.
Even synthetic latex is still “pure” latex because latex just means rubber and both natural and synthetic latex is rubber. The source of the raw materials is what makes it either synthetic, blended, or natural rubber and determines it’s value and performance (along with the type of manufacturing process either Talalay or Dunlop). As far as I know … the latex in the NaturaLatex line is blended Talalay (which is a good quality material).
In some cases this may be true (as he mentioned depending on weight) in terms of pressure relief but it can still make a difference in the support qualites of the mattress and its feel as well. You can see more about the differences between a polyfoam and latex support (or deeper) layers in post #2 here. When you are talking about 6" of latex on top of a mattress … the upper part of this would be more of the comfort layer (contributing more to pressure relief) while the lower part of this would be more of a “transition” layer which contributd both 5o pressure relief and support (to different degrees depending on the body type and sleeping positions of the people on the mattress).
While it’s true that the comfort layers contribute more to the overall “feel” of a mattress … different people are either more or less sensitive to the differences between transition and support layers as well and having more latex can certainly make a difference for many people even if its not as obvious to others. If you can feel the difference (either in pressure relief, alignment, or the overall response of the mattress) … then it would become part of the “value” of a mattress for you. If you can’t feel a difference … then it wouldn’t be as important. It seems from your comments that you can. The firmness, thickness, and type of layering (either polyfoam or latex) in mattresses you were comparing would also make a difference so sometimes it’s difficult to know if you are feeling different firmness levels or the differences in materials.
Like you I would consider this to be quite high even though it has thicker layers of wool in it. there are latex hybrids available (with about 3" of latex on top) for under $1000 so you would be paying quite a premium compared to other similar mattresses.
The value of the Serta Tempo is probably not great because it has only 2" of latex and the polyfoam in the mattress (comfort and support layers) is probably lower density than you mentioned. Are you certain that the polyfoam density is 1.8 lbs (I would want to see the spec sheet that listed it)? Typically the foam used in the comfort layers of Serta mattresses are @ 1.35 lb and the support foam is 1.5 lb). I would certainly want to confirm the density specs. Given the difference in price and construction if I was forced to choose either one or the other I would probably choose the Natura because of the better quality materials and components (if you can confirm the foam density) although neither would be particularly attractive in terms of cost/value. If it turns out that the foam density in the Serta is higher than I believe it is then it may be worth considering even though it has less of the more costly “good stuff” (latex and wool).
If this turns out to be the case I wouldn’t go in this direction either for a latex hybrid.
The wool in the Natura adds to it’s value and performance. The support layers of a mattress aren’t usually the weak link and if they are 1.8 lbs or higher they should be fine.
As I mentioned earlier this would depend on the person, their value equation, and how they felt on each mattress. I would personally choose more latex not so much because it has better pressure relief (which may or may not be the case) but because of it’s other performance benefits but this would also depend on which of the different tradeoffs were most important (including cost) and on whether I could feel a real difference with careful testing. Years down the road you will remember more about how well you slept than you will about how much you paid so within reason the cost is only one part of the “value equation” and better performance (that you can feel) and durability is just as big a part.
Yes … the top 6" of a mattress isn’t just a “comfort” layer but more of a combinatioin of comfort and transition layer. there would be very few if any people who need 6" of any foam for pressure relief along. Even a 3" comfort layer over the right support core could work very well depending on the combination or pressure relief and support it provided and on how well matched the layers were so that the transition between them wasn’t “obvious”.
Since I understand you are in Barrie, I would probably do some research on the phone (if you haven’t already) and call the retailers closest to you that are listed in post #2 here and ask them if they carry a latex hybrid with about 3" of latex over a good quality polyfoam support core. I would also call the manufacturers in the list and ask them the same thing and where would be the closest place you could test it. Some of these would probably have better value than what you have been looking at if they are available close to you so you can test them.
If you are also going as far as Mississauga (which is where nine clouds is located) … then some of the options in the Toronto list here would be even closer to Barrie and some of these would also carry latex/polyfoam hybrids.
I would do most of the work on the phone and if it involved a fairly long drive I would also get prices on the phone (to make sure the drive was worth it) before I went there.
I tend to think you could do better than any of the options you have mentioned so far but again I would do most of my research on the phone from this point and only visit “targeted” outlets which you knew carry something that you know you want to test.
Phoenix