I've been IS38/APR for a few weeks now and have had no issues at all. No CEL's or anything. It's a dream and it's far more power than I need on the street. I've never had my car dyno'd, but I can say it feels SIGNIFICANTLY stronger than ED stage 2. And my turbo sounds much healthier, like it's not being overworked. Pushing it on ED on the stock turbo always felt and sounded like I was asking too much of it.
Jlungo, as for the o2 CEL, when they flash the tune, they can select either APR downpipe or testpipe. Mine is testpipe as I'm running a CTS Turbo catted dp, so I wonder if they chose APR dp when they did yours and that's causing errors.