I have also shot with an IWB rig in USPSA, it's perfectly legal. Not the fastest thing on earth, but it works.
But I have to disagree a bit with Rick, I think you can be competitive with an Uncle Mike's and a few carry-type mag pouches. I've managed to make to B-class in Lim 10 with a KyTac SooperHooper and regular old comp-tac carry mag pouches. It's true that I'm feeling like I want to change my gear out now, but it's gotten me this far.
Don't sweat it, get out there and shoot!