The big advantage to the thicker, heavier mat is that its extra weight dampens the platter better (and that's one ringy mo-fo). The extra weight also slightly increases the flywheel effect of the platter.
I don't have a supermat, but I *do* has a sorbothane-based Oracle Groove Isolator mat, which weighs about the same. It brought a noticeable improvement in dynamic range and clarity.
As for the headshells, the wire leads on the Technics could bear improving. Most places, the silk-wrapped OFC Litz wire replacements are $20, but they're $15.95 at LPGear.
Kevin has a point, that the Technics headshell is only 7.5g, and you would be hard-pressed to find one as light and rigid on the aftermarket w/o spending a lot more money.
OTOH, the 12g Sumiko (or LPGear ZuPreme) headshell increases the arm's overall effective mass to 16.5g, which provides a better match for many cartridges with a compliance in the range of 10-15. The Sumiko/Zupreme comes standard with those $20 cartridge leads, and provides azimuth adjustment as well.
So if nothing else, the heavier headshell increases the range of cartridges that run well on the Technics.
