If you want 400whp with great response than I think you're looking at the wrong turbos. Check out the Twin scroll Dominator 1.5xtr. Even the single scroll unit has great response spooling up very close to stock and will make close to if not more than 400whp on a Dynocom Dyno. You may need to go with this version if you need to stick with a stock DP and exhaust manifold etc. Although even if you went with a twin scroll version your tuner should be able to get it to pass emission standards.
You said you already have a twin scroll manifold and DP though?? So are you not using them? You can't go twin scroll with out them! Either way, I will be going with the twin scroll version in a couple months time and my tuner (a very reputable one) has assured me that in this form the turbo will spool up faster than stock and make close to if not more than 400whp.
From reading your setup it seems like your motor should be good to over 7krpms? Just imagine a 400whp car that spools close to 2900rpms and pulls hard to redline! That sounds like a fun setup!! If you go this root though you will need a twin scroll manifold, up-pipe, and down pipe though. Just make sure you have a cat and your tuner should have no problem getting your car to pass emissions!! Good luck!! Just my 2 cents