Hi tidoubleger,
As you mentioned … I would keep in mind that testing for the “initial feel” or “showroom feel” of a mattress can be very misleading and based on this alone your odds of making the most suitable choice would be less than random chance alone (see this study). This is especially true when you are testing memory foam mattresses which may take even longer than 15 minutes to test effectively so that they have a chance to warm up with your body temperature. I would also make sure that you are wearing loose comfortable clothing and either take or use a suitable pillow when you are testing mattresses so that as much as possible you are replicating your actual “real life” sleeping experience.
If you do test some of the Tempurpedic mattresses then there is a link in the tutorial post to a list of online memory foam retailers or manufacturers that sell good quality/value memory foam mattresses and many of them use Tempurpedic as a reference point that in combination with a more detailed conversation on the phone can help you make the most suitable choice out of the options that they offer.
Some of the better options or possibilities in the Memphis area I’m aware of are listed in post #2 here.
Phoenix