I wouldnt know how fast my 914 is.. I havent driven in it for more than 5 mins.. (im rebuilding it right now)
I would think that a 914 should be able to get very near fast with 150 hp.. and about 150-170 would be needed to be able to stay with a stock 951 or a stock 986.. About 210-250 would be needed to stay with a modifyed 951, or a very modifyed 986 S.
Having driven my dads 944 turbo track car many times, I would say that that car is getting close to scary fast.. with 320hp and 350+ tq, its getting 4.8 0-60.. and top speeds, well lets just say it never ends

. I have ridden in it at 140+ and the time it takes to get to that speed is suprizingly very small.. It just keeps pulling. Personally I wouldnt mind 50-100 more hp.. just for the bragging rights.. (all needed for that is a turbo.. and $$ to buy it with.)
A 914 is for auto x
A 951 is for track