I have the Garrett turbo. It's a Turbo Re-Source Stage II, which is a slightly modified Power Max 2. I love this turbo, but it I really can't compare to the stocker or else. At basically the same time I did the turbo, I did cam, IC, exhaust manifold and intake horn. I was looking to use the smallest turbo possible to make 500HP (which I think I did but I need to get on the dyno). I wanted to retain drivability and spool. To that end, this set up is fantastic. Spools at least as quick as the stocker, maybe better. I can run the smarty on 9 without problems but am using 7 now. I think the stage III would spool slightly slower than the stocker but better than a conventional of the same size.
Honestly, if I had to do it over again, I'd probably just do the ATS 4 or 5000 twins using the stocker. That really is the best of both worlds if you have the $$.