You will love the swap. My 3.7L ends up passing C5's on road courses, so you could imagine what the extra ponies would get you!
Then the guys driving the C5's suck.
I owned a C5, stock for stock a V6 Mustang will not pass one on a road course, not even close, even a V8 Mustang would have a hard time. They are just a better balanced, lighter, smaller and have IRS. And a modded C5 can be a monster on a road course.