Toyo Proxes RA-1 are DOT approved 100 UTQG/treadwear tires. They work quite well, are available in Boxster friendly sizes (235/40/17 and 255/4017), and are affordable.
In the wet, run them with full tread (8/32"). If you're going to run them in the dry, they need to be shaved either to 6/32" or even better, 4/32", otherwise they will slide around like crazy.
Proxes RA1 | Toyo Tires