Hi hmorneau,
A 2" 25 ILD topper would be firmer than the top layer of your PLB Nature so in terms of ILD and the firmness of the actual sleeping surface it would be firmer but you are also increasing the thickness of the comfort layers which would make the mattress feel softer (thickness and firmness work together to create the “feel” of softness) and it would also be softer than the second layer in your mattress which is 28 ILD and is also part of the pressure relieving layers of your mattress that you would feel … especially on your side.
Overall … I think both of them would make your mattress softer (depending on the type of softness you are most sensitive to) but the 21 would of course feel softer than the 25 ILD. this would also depend on the sleeping position you were using to evaluate it and on your specific body type and how far different parts of your body sink into the mattress and the depth of the pressure relieving cradle in each area of your mattress.
When you have a firmer layer over a softer layer it’s called a “dominating layer” which is similar to the type of layering that’s used in the PLB Nutrition (see the specs here) which has 3" of 27 ILD over 2" of 19 ILD and is only a small increment firmer than the Nature which has 2" of 21 ILD on top of 2" of 28 ILD even though the ILD of the top layers would tend to suggest it would be firmer than it is.
Some people would “feel” like the Nutrition was softer than the Nature because of the extra thickness of the comfort layers in spite of the higher ILD of the top layer in the Nutrition.
Phoenix