Hi Thinktwice,
You can read a little more about the effect of mattress thickness in post #14 here. Assuming that the top 3 layers were exactly the same and the only difference was the X-firm on the bottom … depending on body type, sleeping positions, and individual sensitivity some people would feel a difference and some wouldn’t . While you aren’t in a weight range that would “need” 12" of latex … there are certainly some people that would notice a difference and would prefer it.
You can read more about the differences between having a separate topper and having the same layer inside the mattress cover in posts #3 and #4 here and in this topic. Again this would be a preference choice not a “better/worse” choice.
There are too many unknowns and variables involved for me to know whether you would feel “too much” of the firmness of any specific layer in a mattress or how the transition between different firmness levels would feel to you whether it was inside or outside of the mattress cover and the only way to know this would be based on your actual testing or sleeping experience on a mattress. Different people can have very different experiences on the same mattress or mattress/topper combination.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
In the end you are the only one that can feel what you feel on a mattress and again there are too many unknowns, variables, and personal preferences involved to be able to use a formula or for anyone to be able to predict how a combination of layers will feel to you or to make a specific suggestion or recommendation about which mattress or combination of materials and components would be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing or personal sleeping experience … hopefully using the testing guidelines in the tutorial post (see mattress firmness/comfort levels in post #2 here).
Phoenix