Talk to the fine folks on
www.rennlist.com in the 944 Turbo/S forum. I don't like linking to other forums, but these guys know how to get you a 500hp 951.
I assume they'll refer you to either
www.lindseyracing.com or
www.vitesseracing.com
As for 700-800 hp monster, not yet, I've seen some close to 700, many around 400-500, but the stuff to get 700+ hasn't really been developed yet, be prepared to spend somewhere around 40K to get it to those numbers.