Have you tried the Mazda Furai? Any success?
As
@hall90 mentions, you should be able to win the GT Championship with the Furai.
The Furai is a little top-end speed challenged, so for a high-speed track like Spa, you may want to purchase a horsepower upgrade to give yourself more speed.
Its fast enough to easily beat the AI on the twisty tracks like Laguna Seca.
Upgraded to about 650PP, the Mazda Furai should be fast enough to win at all five tracks in the GT Championship.
Good luck!
GTsail