I'm doing a build with a similar turbo, 6758 v-band EWG. It's a single-scroll turbo but I'm running twin-scroll Impreza headers and making a customer rotated pedestal. I'll be running my car in with a VF36 however while I get stuff fabricated.
From the results I've seen in my own car and in speaking to others with very similar setups, VF36 will max out around apporx 240kw on 98 and approx 280kw on Ethonol.
In regards to the 6758 for my motor, 2.1L, I'm expecting to make some decent power as I'll be running it at some pretty high boost levels with a balanced motor that flows a lot of air. A friend of mine is running this on Hyabusa motor, 1300cc, on 7psi making well over 300kw. This is dedicated track car however, so I don't really know if it's comparable.
While my friend and I have identical 6758s, they are not the 'off-the-shelf' variant. They've been modified to run in the Indy Car series so I don't know if they flow any differently or what have you.
However, buy a turbo to suit a) your driving style and preference b) power goal. There is no magic turbo, so like most people, you'll have to sacrifice one or the other to find that happy-medium
Re: your comment above about "boost tapering off" - are you saying the boost line is not horizontal all the way after hitting the target boost level? I can understand torque and power tapering off but if your boost is taping off wouldn't that be a leak or something?