Have you tried using the Lotus Elise 111R? I'm not a spectacular driver and I was able to win Gold repeatedly with similar cars following a similar formula -- keep as much weight off as possible. The Lotus is so light, with further weight reductions, that you can literally just throw the thing around corners and it has plenty of power to get around the exotics driven by the A.I. Give it a shot if you haven't.
I think the 458 has too much weight and more power than needed on this type of track. If you were starting from a standing grid, no problem, but you're playing catch-up on AI that is half a lap ahead of you by time you cross the starting line.