Joined: May 2016
Location: Lake Ariel/Holland
The dealer cannot void your entire power train warranty for headers. If they find the headers caused a problem with the power train, then they can void that part of your warranty.
The bigger problem with LT headers is they move the Cats. That is illegal on any public road. Depending where you live this may/may not matter. I had LT's, a S/C and Meth injection in my last car and it would pass emissions every year because it was not throwing any codes.
Typically to get the most of LT headers you need to tune the ECM and that WILL void your power train warranty IF the dealer needs to authorize repair work from Ford.
I've never had Mid Length headers on a car so I can't comment. If they don't move the cats, they should be OK. I thought only shorty headers did not move the cat position.