Hi jak8,
Only you can “feel” what you feel, so the only way to know whether any specific mattress design or combination of layers and components is a good “match” for you in terms of comfort, firmness, and PPP with any certainty will be based on your own careful testing and/or your own personal experience when you sleep on it. While you didn’t mention the complete specifications of each mattress you are considering, and which mattress upon which you are considering placing this topper, in general adding only 1.25" of Talalay latex on top of a mattress would be considered a more “conservative” modification of surface comfort and generally wouldn’t be in the arena of “overkill.”
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”. Changes in the deeper layers tend to have more of an influence on the overall “support” of a mattress, and your proposal, while allowing for a bit more surface conformation, would in general have a more minor impact upon overall support and have a greater impact upon “pressure relief.”
Phoenix