take what ever part you buy, and force the stealership to install it. that way, they have to warrant their parts!
also, how are they going to know that you swapped TSX cams with your Si cams? they'll only know if A) YOU tell them, B) they use a micrometer to check the duration and lift of the cams. but what are the odds of that happening?
if you are worried about warranty, do go to the extreme with your mods.
for example, do buy a stroker kit and then have some noob for a technician tune your car and have the valves hit the pistons etc etc
i got my dealership to put my suspension on my car before delivery, and its covered.