Well, I think the F50-GT is better still, but the SP campaign scales to what you select. If you pick your Enzo, you'll be up against other Enzos and similarly-specced beast cars. The best way to game the really hard races I found was to take your favorite car in the E class (the Evo for me). It's so much easier to drive better at non-Enzo speeds.
On Gold, you really have to strive for perfection on the cone challenges (going fast, not hitting any cones, keeping your combo alive from start to finish). On Platinum, you need to be perfect AND creative. The races of course are just much more competitive...you'll typically see one CPU car zoom ahead of everyone from the start and you'll have to really work to catch him over the course of multiple laps. I had a lot of photo finishes on those races.