Since there are plenty threads all over here, I thought I might make this into one discussion to make things easier for those who have those problems :
threads I seen so far :
Very low FPS - by Rydberg123 - xBox360
Legit copy on 360 - Annoyed by FPS : by Tom7792 - xBox360
FPS PS3 Slim - by hhamzah99 - PS3
Improving PS3 Performence - by Southparkmayor - PS3
Basically the issue with what I've seen so far is occasional FPS drops while playing the game, discs are installed as recommended and issue persists,
personally what I'm experiencing is that every now and then, mostly when you drive or walk into junctions or even look around, FPS sometimes drops by 10-15 FPS, feels around 20 even lower, the game sometimes runs smooth, obviously when the place is less crowded, but thats not like what people are reporting around the net.
I've seen this with PS3 users aswell
Heres a comprassion between consoles so anyone experiencing this can get the rates both consoles have
Solution 1 (360) - Clear XBOX Cache from storage. Dashboard --> System Settings --> Storage --> Press X, Clear storage. might work for some, didn't work for me.
Please if you have this issue, make sure you specify which console you use.
/discuss and good luck