I'd be in favour of race upgrades, but only sensible ones. For instance; a high-downforce upgrade for the GTR cars. Could be illustrated with added diveplanes, and larger wing endplates. Advantage would be increased downforce, disadvantage would be increased drag. This would be something only run on tracks like south city and fern bay where top speed is not as important, and the upgrade would be 'un installed' for tracks like Aston and Blackwood.
Also, larger brake ducting for tracks like Fern bay and South City. Advantage is cooler brakes which won't wear out as fast, disadvantage is increased drag.
Ideas like that I could support, although maybe not using the credit system at all as it would be an annoyance. Using credits for any must-need items like fuel and the like is a definate -1 for me.
That way its not just a "buy the best" like in NFS:U, it is a purposeful upgrade which does not suit all tracks/drivers. This sort of thing is done in the real world too (especially in F1 and LeMans type enduro racing).