lazs,
Doesn't take 400hp to mess up a power shift... I looped my firebird in a 1-2 power shift, and it only has about 320-340hp. Dunno exactly how much hp it has because although GM rated it at 305, most owners of the same car are getting around 295 at the wheels on the dyno, which means it's making much more than 305 at the crank.
In any case, even a stock LS1 f-body will loop during a full power shift if you're not really careful or if anything isn't quite right (uneven surface, one tire a little worn, etc).