2005 (F50?) sway bars

[StarPD]

As noted in another thread, my new (to me) 2005 Q45 with 20" wheels and crappy Sumitomo (probably the cheapest the dealer could get for a used car) tires understeers pretty badly in tight fast curves. I intend to replace the tires, probably with BFGs, Goodyears, or Bridgestones when I recover from buying the car. In the meantime, I think I'd like to put a thicker rear bar on it.

Does anyone make such an animal, or is there a stiffer one from another vehicle that will fit without blacksmithing?

Suggestions for tire are welcome too, if anyone is using 20" wheels.