I just gave the Saleen supercharger as an example because I think its such a complete kit and a great street setup for such a good price. It can be had for under $3200 for the 99+.
Allen Engine Development (AED) has released an intercooled setup for the 99+GT's also. Its another Eaton setup that makes a great street package IMO.
AED supercharger
My friend A-train has a stock 99GT engine in his Tbird running and AED and he has gone 12.8's. Figure thats an auto car running about 400lbs more weight. So there is potiential for a mid-low 12sec Mustang with one of these.
Of course, if you want to look seriously into blowers, you should not leave out checking into the big names like Vortech, Paxton, ATI (Procharger,) etc.
So, to reiterate, 310-320Hp will take a ton of bolt on's and it will just be much easier and much more of a streetable package if you went nitrous or forced induction.