Hi excalibur5,
Thank you for the clarification on your set up. I read it as you only had a total space available of 2"-3" for your mattress, as opposed to 5"-6". :S
Hopefully your polyfoam piece is of a higher density (2 lb or above would be nice) and a firmer ILD (mid 30s or above), but I’m guessing you may not have that information at hand. Regardless, I know you are thinking of keeping that as your base layer, which can be a good idea if it provides you with adequate support.
At your mass, I would recommend 3" of topper material, and I would avoid going “too plush” for the latex (teens-low 20s). You’d probably be best served with something in the more “medium” range of plushness (mid 20s ILD or so) on top of a firmer piece of polyfoam being 275 lbs. Of course, everyone has their own individual comfort preference, but my concern would be choosing something “too plush” and “feeling through” that to the layer of polyfoam.
There are some different vendors in the component post here who supply latex as well in case you hadn’t already found that post.
Excellent choice!
Phoenix