Why would you want a top mount manifold? I've never heard of anyone doing that for a bolt-on turbo.

Performance, I wouldn't expect much from it. Maybe just a little better than a S15 t28.
As for the turbo choice, the 3037 would be awesome! It's a very expensive turbo though. As for it's drivability, sure it will build boost later then a t25, but it's by no means a lag-queen.