You think a Vortech V3 Si Trim will make 20 lbs on a built S65? No way. Not a T Trim, either. Maybe a YSi, so then you probably need custom brackets in addition to a custom tune and maybe better intercooling than what is out there now. Custom dedicated 10 rib belt drive system. Probably race gas also. A stronger DCT trans. A heavier duty 6MT clutch. Sounds like a very expensive project. I like to see other people do projects like this.
I personally would not bother with the centrifugal were I going for 800-900 rwhp. Turbo is the way to go for big power from a relatively small engine. I have had both on my 99M3, a Vortech T Trim running 16 psi and a turbo running 20 psi, but that car is a lot easier and cheaper to mod than an E90M3.