Hi rdr93,
I agree that foam softening isn’t the ideal way of making a mattress softer. It will also soften unevenly and the parts under the heavier areas will soften more than the parts under the lighter parts of the body. The 1" layer in the Savant is not quite as bad because it is already soft and there is only an inch of it so further softening won’t have the same effect as the softening of a thicker firmer layer. They are the same quality foam but the 2" layer in the genius is much firmer (this is the layer that gives the mattress a firm rating).
Yes … the deeper in a mattress or sleeping system a layer is the less it will be regularly compressed and be subject to wear and tear and the longer it will last. The only challenge with this is that you wouldn’t want the topper to be so thick that the total of the soft foam is more than is suitable for best alignment for your body type and sleeping positions. In the case of the Genius though … an extra 2" should be fine but I wouldn’t go thicker than this because then the softening of the polyfoam and memory foam would have a greater risk of putting you out of alignment (the foam under your heavier parts will soften more than the foam under your lighter parts).
There really isn’t a “formula” where you can plug in weight and height statistics and come up with a specific recommendation that would apply to a specific person because there are so many variables involved both in different body types (even in the same height and weight) and with the many different variations of sleeping positions and preferences and even different physiology and more or less movement on a mattress (asleep and otherwise). There are also no “standardized” specs for memory foam except for density and in some cases ILD (which is softness firmness but this is a very misleading spec with memory foam) and there are hundreds of different memory foam chemical formulations (and more coming out all the time) which can alter some of the many specs that are part of a memory foam formulation (you can read more about this in post #9 here). Memory foam is tricky because it has many more variables than polyfoam or latex and many of the most important specs aren’t standardized or even disclosed which is why personal testing can be important.
Having said that … there are “averages” that apply to many people and if you can describe the general qualities you are looking for a good memory foam supplier will “match” what you are looking for with what you want. In your case you would probably be looking for more open celled (cooler) memory foam that has a slightly faster response and less temperature sensitivity. There are more 4 lb foams that have these characteristics of a more open celled structure, faster response, and less temperature sensitivity in a lower density foam … but the tradeoff for lower density is durability. If you choose 5 lb foam that has these characteristics … then you can have the same benefits with greater durability but generally at a higher price. A topper also has the advantage of being replaceable when and if it wears out faster than the mattress itself so it can be OK to choose a slightly lower density because it can be replaced. This is what hotels tend to do (add toppers or other bedding packages) to extend the life of their mattresses.
To find the best choice … the retailer or source you choose and their level of knowledge can be the most important part of your buying decision because buying from someone what doesn’t have the knowledge to match their products to what you need can lead to choices that may not be suitable for what you are looking for.
With your height/weight and sleeping position (assuming side?) and based on averages … a 3" - 4" pressure relieving cradle is generally what you would need and with memory foam … because it tends to be softer (when it is warm) than other types of foam and you can sink into the softer versions more deeply and more evenly … then a little extra (say up to an inch) is usually OK compared to other materials. So if you add a 2" memory foam layer to the 2.75" that you already have in the Genius … it will be similar (in terms of the memory foam anyway) to the Savant and would be in the range that should work well for you. Other sleeping positions would do better with thinner layers.
So I would talk to the 4 memory foam sources that are listed in the post I linked earlier and tell them what you want and then the one that has a combination of the properties you are looking for, the density that gives you the durability you want, and the price that is most attractive is the one I would buy.
Hope this helps.
Phoenix