Hi 01svtL,
Yes … their ratings are based on the overall “feel” of the mattress and not just on the ILD of the latex. While comfort ratings are always somewhat subjective … they both have different covers and quilting which would affect the “feel” and relative softness of each.
In “theory” yes it should make a “little to a fair” bit of difference (based on the guidelines I use for toppers in post #8 here). The extra 2" of softer latex will “allow” your wider shoulders to sink in more and improve pressure relief and is still thin enough to have the least possible effect on alignment.
A little thinner is much “safer” in terms of risking alignment because comfort layer / topper combinations that are too thick can be great for pressure issues but are riskier. “Just enough” and no more is generally the best approach.
While only your own sleeping experience can know for certain and I didn’t speak to you on the phone so I don’t know all the information you shared … it’s probably very close to the choice I would have suggested as having the best odds of success as well assuming the amount of extra softness you wanted is in the “little to a fair bit” range.
Phoenix