I think they're pretty fast. However your mach has stock 305? STi something around 290-300 max (their ratings constantly change - apparently due to the self tuning it does). Your mach should "handle" them.
But you can't beat this: Midpipe + Catback + Off the shelf tuner = 50 actual, guaranteed horsepower. FIFTY.
You can't beat it's handling. Suspension tech wise it's simply in another playing field than a Mustang.
Sorry I say all this because I have both a GT and an 06 STi. STi with the mods stated above, and it absolutely owns. Off the line is sweet, a straight line is pretty sweet, around the corners going as fast as you want is unrivaled.