I did the dynamometer development work on the early (5.0 L) Mustang GTs at Ford R&D. On the dyno shortie headers were about 5 hp less than long-tube headers when run through a compete exhaust system, this being on a V8 engine in the range of 225 - 245 HP. I agree with DeuceCoupe that on a street driven car split iron exhausts are the most solution between performance and durability. I use the split, cast iron factory EFI exhausts on my 300 street hot rods.
