Hi Caribou,
I’m not sure where you read this but the thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful because whether a thicker or thinner mattress would be better or worse for any particular person will depend on the specifics of the materials and components in the mattress (type, firmness etc) and on how all the layers and components in a mattress interact together and with the person sleeping on it. The thickness of individual layers or components or the mattress as a whole is only one of many specs that are used to make different mattresses that perform and feel differently from each other and that makes a particular mattress suitable for one person and not another.
The height or thickness of a mattress by itself also makes very little difference in terms of quality or durability and is always much less important than the quality and durability of the materials inside it. If a mattress is thicker but uses larger amounts of lower quality and less durable materials than a thinner mattress then it would be less durable and can often be a problem waiting to happen with premature impressions and foam softening. This is one of the reasons there are so many durability issues with many of the pillowtops and eurotop mattresses that are made by the major manufacturers. Thicker layers of lower quality materials can add to the cost of a mattress but can often cause even bigger durability issues than thinner layers of the same lower quality materials. In your case there are no lower quality materials or “weak links” in your mattress.
If a mattress uses only high quality materials then in some cases and for some types of mattresses a thicker mattress can be beneficial for some people … especially for higher body weights … but this is only if the mattress uses good quality materials that won’t soften or break down too quickly and there are no weak links in the mattress. There is more about the effect of thickness in post #14 here but there is nothing that is inherently “better” about a thicker mattress unless your testing clearly indicates that a specific mattress is a better match for you in terms of PPP than a thinner one.
I don’t recommend any specific thickness for anyone because the most reliable way to know whether any mattress is a good “match” for you in terms of PPP is based on your own careful testing or personal experience (hopefully using the testing guidelines in the tutorial) and while it’s true that with component latex mattress for those that are approaching 300 lbs or so they may do better with an additional layer of latex and a slightly thicker mattress … your mattress isn’t an all latex mattress and you aren’t close to the weight range where “greater than average” thickness would be an issue even with an all latex mattress so if you have tested the mattress carefully and are confident that it’s a good match for you in terms of PPP then I certainly wouldn’t be concerned about it’s thickness.
The flexible base underneath the mattress also acts as an additional “active” layer in the mattress as well so this would also be a part of the total “sleeping system”.
In any case … it sounds like you made a high quality choice … and congratulations on your new mattress 
I’m looking forward to your comments and feedback when you have the chance to sleep on it for a bit.
Phoenix
Phoenix, thank you so much for that incredibly quick and very reassuring reply. I will stop worrying about this now! And yes, I will come back to tell you how the mattress works out for us.
Thanks again for your help! It’s so appreciated.