I left the stock mid-pipe on when I put my Stomung exhaust on, and it seems to work pretty well together. Just doesn't seem like it would be worth the money for the midpipe to free up maybe 1 horse, especially at the risk of losing low end. Actually, I'm not even sure what size the stock mid is.
I have a 2.5" catback and have had barely noticeable low end loss, if any. Top end feels a bit better, but isn't earthshattering I doubt the gains would be all that much different from the 2.25" catback. I bought the 2.5" thinking it would compliment future FI mods better... and now I'm selling my car... lol time for a wagon to haul all these parts around for my business... possibly one with an STi drivetrain swap