Hi suffolk,
You certainly did well to get the specs you did 
Overall there are no obvious weak links and I would say it is in a good value range.
It would be very difficult to replicate or make comparisons between the Brooklyn Bedding Ultimate Dreams and the Natural Elegance other than by your own testing and experience based on “feel” because they are completely different designs and the comfort layers are different thicknesses and have a different mix of materials.
Polyfoam ILD is also different from latex ILD because they use a different layer thickness to test ILD in each of them and polyfoam will be firmer in the same ILD than latex. I would guess that a rough equivalent to 19 ILD polyfoam would be in the range of 24 ILD latex although there are also other differences between the two materials and using ILD alone as a way to compare different materials can be quite misleading because of the other differences between the two materials.
The different quilting layers will also make a difference in the surface feel of each mattress and will also affect the compression of the layers below them.
Finally a comparison would also depend on your weight and body shape and sleeping positions because there is an additional inch of softer foam in the comfort layers of the Natural Elegance and layer thickness can make just as big a difference in how a mattress feels and performs as layer softness. The layer below the latex in the Ultimate Dreams is the firmer support core so some people would feel the difference more than others depending on how deeply you sink into the mattress in each area of your body and which of the layers you are most sensitive to.
If I was forced to make a guess though … and assuming that the quilting layers were roughly equivalent (which isn’t a “safe” assumption at all but this is only a very rough approximation anyway)… then I would probably use a 24 ILD in the BB mattress as a rough comparison between them although as I mentioned this would be more accurate for some people than for others depending on your body type and how you sink into each mattress.
If I was to go in this direction I would certainly wait until I had ordered the BB mattress and then use your personal experience to decide on the thickness and ILD of a topper rather than going by specs. I would also consider that adding a topper that you haven’t tested in person in combination with the mattress you are sleeping on is always an unknown variable and can sometimes be almost as difficult as choosing a mattress. I would also err on the side of firmness because it’s much easier to make comfort layers softer by adding a topper than it is to make a mattress that is already too soft any firmer because this usually involves removing and replacing layers rather than adding to them.
I also place a high value with any online purchase on the mattress or layer exchange policy for the mattress and your recourse in case you make a wrong choice because you can use your own sleeping experience to make adjustments unless you are very confident in your comfort choice. This and the higher quality materials including the wool in the quilting would make the Bamboo Bliss a strong contender for me (based on my own preferences which may be very different from someone else).
All of these would be good choices though in their own way and once you have eliminated your worst choices and are down to final choices between “good and good” (which you are) … then only you can decide on whether any of the differences between them are “worth it” based on which of many objective, subjective, and intangible parts of your personal value equation are most important to you.
Phoenix