I just put aeros on (more for fun) and soft tires and the GT was a breeze. Do a friend without money a favor and send him a Citroen, that was even easier. I'm glad I didn't waste money pimping the cars out, all they needed was tires... (though I did add a few HP on the non-shiftable Prius), and too much HP would remove any sense of accomplishment... ah nevermind. Just bogus.
Problem is... you don't really even need money anymore in this game. I too hope the seasonals go back to the awesomeness of the GTR challenge, but add in damage and shortcut penalties.