Hi mwilson,
There is really no should or shouldn’t here and each person will have a different idea of how close one mattress is to another one when the thickness and layering is different. Some would say it’s “just like it” and other would say its “different”. The top half of the mattress is the part that most people (but not all) feel the most and the Beautiful has 3" x 21 ILD and 3" x 24 ILD which would be a fairly risky construction for you IMO. It is not uncommon that someone loves the “feel” of the Beautiful (or any mattress that is too soft) in a showroom but then discovers it wasn’t quite as good for support/alignment as they hoped. Don’t forget that comfort is what you feel when you first lie on a mattress but support /alignment is what you feel when you wake up in the morning and it takes more careful testing and listening for more subtle cues from your body in a showroom to check for this.
When you are ordering a mattress that has exchangeable layers online I would build in the expectation of a layer exchange … even though the odds are good that you won’t need it … so that you can be more comfortable with your initial choices. There is no way to predict for certain what anyone will feel on a mattress they haven’t tried in person except in approximate, theoretical, or “average” terms and the only way to know with absolute certainty is to sleep on it. In most cases people tend to put too much emphasis on exact ILD’s or exact specs when most people have a wider range of layering that would work perfectly well for them. There are really only two or three basic initial configurations with a 3 x 3" layer mattress (S/M/F or M/F/XF or M/M/F) and in most cases one of these fits almost everyone or at least is close enough that re-arranging the layers or a single layer exchange will be just right.
9" of latex will feel somewhat different for many people than 12" of latex (if the layering is similar) but more for heavier people than lighter ones because thicker mattresses have a greater range of compression (see post #14 here). So the question becomes is one really any better than the other or are they just different. In much higher weight ranges or for particular circumstances or preferences then thicker can be “better” but for most it is just “different” and for some it isn’t any different at all if the top layers are similar. Everything depends on the person and the individual circumstances, preferences, and sensitivities and there are no real answers to much of this outside of your own personal experience. In most cases 9" is fine and will be “similar enough” to another mattress that uses similar layering in the top half of the mattress if the bottom layer has a similar firmness level.
Yes … to me the approach sounds reasonable. There is no “down side” to a thicker mattress as long as the layering is appropriate but it would depend on how sensitive to any differences you believe you may be and on the importance to you of “matching” the Beautiful more closely in specs … for better or for worse … rather than “targeting” your needs and preferences.
Perhaps a simpler way to answer this is that the 10000 in the layering Shawn is suggesting would most likely provide you with “enough” pressure relief, “enough” support/alignment, and feel great for most people with some backup in case it “wasn’t quite right” for what you need or prefer. The “feel” would be different from the Beautiful however. The closest you could probably come to matching the Beautiful would be a 13000 with S/S/F/F but I would question the wisdom of doing that because of the potential risk of support/alignment issues with top layers that are too soft/thick although you would still have a “backup” available by exchanging layers if that turned out to be the case.
While latex is more “forgiving” than other materials because of its ability to be soft and supportive at the same time, it’s not immune from the effects of choices that are too soft. If you did need do a layer exchange to improve the support/alignment to “correct” the layering, then you would once again have a mattress that wasn’t the same as the Beautiful but would be “enough” in terms of pressure relief and support/alignment and your preferences of “feel” and you may “like” it even better. Is the risk and the importance of “matching” another mattress that may not be the most suitable for you in real life instead of targeting your needs and preferences in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences) or the potential benefits of a thicker mattress or the ability to fine tune it in more ways worth the extra money you would pay for the 13000? Your answer to this will decide which is the “best” direction for you.
Phoenix