Hi PMC_MTT,
As far as the latex itself goes … they would be the same (assuming they were the same type of latex) whether they were inside a mattress or in a topper. Of course the cover material could be different.
It may also be more difficult to find an actual topper that is as firm as a typical bottom layer in a mattress. There is a list in post #4 here of some of the better sources I’m aware of for mattress components including latex layers.
Your “plan” of first trying two layers and then adding a third based on your experience (choosing the thickness and firmness level) can be a good one. I would also make sure that the cover you use is good quality ticking because a thin mattress cover can expose the latex to ozone or ultraviolet light that can break down the latex prematurely. If your original layering doesn’t have a cover I would also make sure that you don’t expose the latex to direct sunlight or any of the other things that can soften or damage it (there is a list of things that can break down latex here).
Phoenix