If you mod your car for the purpose of thrashing the sh*t out of it to beat the bloke next to you at the lights in his modded WRX then it's fair enough you lose your warranty claim.
If a dealer tells you they won't cover anything that happens as the result of a non-genuine part, think of the implications for other makes such as Toyota, Holden, Ford etc where you pick up generic parts from KMart etc.
Sounds like "third line forcing" to me.
2012 BMW F10 520d M-Sport
2010 BMW E70 X5 xdrive40d