as most manufactures have a Performance aspect to the company modding a vehicle won't impact your warranty (let me finish).
First, ask your dealer, and call a few other dealers, some dealers are specialty dealers that may not care about mods. Where i live, 1 dealer in the whole city is SVT authorized. 1 dealer is Saleen. 1 Dealer is Roush. 1 Dealer is FXR. the rest don't do performance.
second. alot of times warranty still applies IF your modification was a direct cause to something failing. You put an ungodly, gear ratio in your F150....Lifted it 18"....put on 54" boggers on 30" rims and blow your diff.....No, sorry. you caused the failure....no warranty.
You put a BOV of you ST and the rear diff blows.....that mod was irrelevant to the failure and a warranty claim would most likely be approved.
Depending WHAT your modifying, i don't think many dealers will have an Issue....Wheels, bov, CAI, exhaust, intercooler i think are safe bets, BUT double check with local dealers.
A tune is tricky...which is why a plug and play tune is better....something that can be reverted back to stock easily to get the warranty work done, and then put the tune back. Again, as previously mentioned here....a reputable tune place (ZFG, Palm Beach) could also be asked prior to getting a tune about whether it effects the warranty or not. Not to mention those places aren't maxing out components to get the highest number for a dyno....
I'm in Canada so it might be different....we just have to say sorry at the dealer and our stuff is covered under warranty
hahahaha jk.