Simple answer: if you are in anyway worried about warranty, don't mod the drivetrain. Regardless to the quality of the mod, when major problems occur, BMW can turn around and say anything they like and it's up to you to prove them wrong. Are you willing to go through the hassle?
Also, no remap is invisible, even if it's removed before going to the dealership. A casual diagnostic will not show any signs of a remap but if BMW wants to find it, they will.