The engine is pretty much a standard GT engine with an Eaton M90 blower.. 15psi from a stock setup is kinda pushing it, I would bet that your gauge is off 7-8psi at wot at 5000ish rpms would be more realistic.
The pulley on your blower looks stock, one good mod would be to basically spin the blower faster, either by putting a smaller supercharger pulley, a larger crankshaft pulley or a combination of both. I know that the M90's can't be spun too fast without loosing power. You said that it doesn't look like it has pulleys on it, if you decide to change them keep in mind standard overdrive pulleys on a supercharged car will lower boost, so the idea is to spin the supercharger as fast as possible to the point that it looses efficiency.
Check out
http://bluegt.com/.. its a website that a guy made of his car that shows how his car has changed over time including a saleen series 2 blower..
I would say that 310-330ish rwhp is probably what you'll see out of the stock supercharger. If you are looking for higher numbers an aftermarket supercharger of almost any type will take you to 380-400rhwp at 10-12psi. You could run 450ish with a little headwork and cams, but the standard max for a stock bottom end 2v 4.6 is around 450rwhp. After that you're looking at a complete engine build, and an entirely new fuel system.
You might add "water wetter'' to your heat exchanger system to try and cool things down a bit, make sure the fans eventually do come on too, if not they need to. Heat will be one of your biggest problems with the eaton and overdriving it so keeping things cool will be important.