I used the Ferrari 458 Italia and it does the job up to the All-star event. But in this event, the Race Cars are much quicker and you have to be more than perfect to win. If a Race car overtakes you, there is almost to chance to regain the lead.
The Corvette ZR-1 RM is a much faster car than the 458, but importantly, you have to buy the RM kit. The RM mod gives you the aero package with wings.
I like to use Suzuka to tune as its a track I driven a lot and it has all types of corners. My 458 is reasonably tuned and I can drive Suzuka at 2:00.5 lap time. My ZR-1 RM is not well tuned yet, but I have no problems getting it to a 1:52.4 lap time, 8 seconds faster than the 458 Italia. It's in a different league.
I tested the ZR-1 RM in two of the All-star races, and performance is on level with the race cars. I even let one past me for fun (a C9 i think), and there was no problems to overtake it again and win. The ZR-1 RM is clearly a good enough car to get all Gold in the All-star events.
My conclusion. If you cannot afford a Race car (I can't), the ZR-1 RM might be the best car out there. It's about 600-700K credits with all mods including the important RM mod included.