I am also in agreeance. I don't really want to put a lot of money into performance mods for my 4.6L. My reason though is not because of warrantee issues, but because i already have two 5.0's that are pretty well modded. Instead of putting money into a blank canvas (the '03GT) i'd rather put it into a car where i already have a solid base to build up on. Not to menton 5.0 mods are a lot cheaper than 4.6 mods
Anyway, here's my little list i have been working on for my car.
Appearance
-SS bumper inserts (installed)
-Interior Upgrade Package (still need shifter trim and pedals)
-WHite Face gauges
-20% tint
-Mach 1 chin spoiler
-Mach 1 grill delete
I would say skip the Mach 1 chin spoiler and get a 03 Cobra front and rear bumper, they look excellent compared to the stock. Plus I doubt it's that expensive.