I really don't think the exhaust is gonna have much to do with the low end power loss. At most, the exhaust leak is going to effect noise levels, as this isn't a tuned header we're talking about. I think it would be more of an ACIS problem, timing, or just a dire need of a tune-up. OR all of the above....
To the OP: Have you been in a good running n/a Supra so you can make a good comparison?
For n/a exhaust, here's a good read:
http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html
It's for v-8 Chevy's, but the basic science still applies.