Custom soundtrack support? I don't see why they wouldn't include it. So many games allow it now.
Playstation games can't do this as Microsoft gave Sony seven years to implement it but they never did so Microsoft finally just decided to patent it for their console/future use.
I used to be a big fan of Granturismo, I still remember playing the original and finally obtaining the GTR Twin Turbo(Fastest car in that game) and just having a blast. I actually have GT3&4 but never played much as they were still basically the same games just more cars and stuff.
That's why I turned to Forza as it has more things to do, plus car damage and custimization, online, etc. Best car game Ive played though was probably still Mario Kart or something as they are so much fun.. Or Test Drive Unlimited, awesome game.