judging from other posts in the forum (search is a great feature ) it appears that anything other that the retail could produce problems, especially in system linked games.
That's the whole point of etiquette. If the host is using a lesser car, then match his car. If you want to use a better car, host your own game. I for one like to race something other than "Ultimat ...
I was playin' against someone that had a faster rally car that wasn't unlocked for me yet and despite how fast and perfect you do a course there is no way for you to win. So pick cars of similar perfo ...