If you've already got a feeling 400 won't be enough, you may as well do a cam up front then. IIRC the stock cam turbo record from back in the day was low 400s. Certainly it could be pushed further with current tech, but no reason to restrict yourself as a cam is cheap as long as you've got it all apart.

The 6262 that comes with the z3/Z7 kit still has some headroom at 400 whp. I would think it could be pushed to 500+ whp, but of course depends on other engine mods.