Another wrinkle to add, using your example of the Q400 is lets say you buy the car and are quite happy with it for a year or so.
But say after getting beat a few times you want to turn up the boost....sure, if something happens you may be able to get it replaced by the warrantee but it may be voided as well.

My point is, that even if you get a car modded quite nice already from the dealer, in time chances are you will have the urge to change something, and then you deal with the possibility of voiding that precious warranty that you paid so much for.

Personally, I enjoy the work in progress aspect of the car, seeing how well things go peice by peice, but I can also understand the attraction of buying a already finished car.