Hi Lynne,
I switched your post to this thread so I could keep them together and read your other posts without mixing up threads (and me :))
I agree with you that component mattresses certainly provide good options and can make the higher risk of an online purchase worth it when the local value available isn’t equivalent (which you can decide on when you visit the other places you are planning to visit). Different designs also have differing abilities to customize before during and after a purchase which is also part of their value.
In some configuration its certainly a legitimate probability yes. The key though is not just in the design (6" + 3" plus cover) but in the different layers and firmness levels that are in the design and how they will interact with each other and with you. The two ways for you to decide on this are first of all your own personal testing on similar mattresses that may be available locally and if they are “apples to apples” comparisons can provide a good reference point for you. The next best way would be to go with the suggestions or insights provided by each manufacturer or retailer who can give you more specific information about the designs components, and options they have available (including the cover they use) and can provide you with information about the “averages” of their customers that may be similar to help you decide on the layering that you believe has the best odds of working well for you. Their suggestions about their own designs and mattresses are much more valuable than any “theory” I could provide. My goal is to help you with “how to choose” not “what to choose” which I leave to more detailed discussions between you and the manufacturers and retailers you are dealing with.
You can read more about the different types of latex in this article post #6 here and also more detailed information comparing blended vs 100% natural Talalay in post #2 here.
Phoenix