Well my friend had won this race but sold the RX8.. I redid the Amature NA race (all 3) and I only won 15k in each race and at the very end I never got the RX8 ...
Unfortunately, because the NA race isnt a series you can only win the prize car ONCE, in this case the RX8...
you can race it again and get the prize money but not the prize car...not to worry though, this car can also be won on the Amateur - Gran Turismo World Championship....but its one of 4 possible cars..
Once you win one of the three race races once you can't ever get the prize car again. However the race series, the ones with the enter series options, you can win over and over and over.
I've never used the Rx8 but I assume you can win it in another race series....check the front page of this site for prize car list in various leagues.
You can win the RX-8 in the Amateur GT World Championships, it's a 10-race series and using save/relaod is a great way to win the RX-8 in one of the two "hidden" colors: "Black Mica" and "Evolv Puaple"
If your looking for a truly unique color choice, then the purple one is worth your time and effort.
Otherwise the only way to win an RX-8 in it's standard colors is to get all gold in one of the license tests.