I used the first car I bought in the game, the J-Ltd, '91. Sure, I tuned it up so it could pass the others, but also tuned the suspension so I could take the big turn while minimizing understeer and oversteer. It was fun doing that.
4 hours is 4 hours. There is no car in the universe that will make that pass more quickly.
All the MX-5's in the game are capable of winning this race. It's up to you to do it in the way that satisfies and amuses you the most.
Respectfully,
Dotini