Well, after doing a bit of research, including this forum, I finally bit the bullet and bought my TPMS system for the TT. Went with the TST 507, after exploring their internal as well as their flo-thru model. Four aspects sold me on the basic system after speaking to their customer service representative, who, by the way was extremely knowledgeable on their product line. One, no need to replace my existing rubber valve stems with metal. (Note that the flo-thru requires the metal stems.) Second, the lower profile as well as weight of the basic 507 system. Third, the ease of changing the batteries, which are available anywhere. Fourth, the reputation of TST with respect to customer service.
Price for the basic system, direct from TST was $233.00. Should arrive in the next couple of days. Can't wait to test it on our spring road trip in March.