Do you know what the efficiency range of the blower is and what it's spinning at with the current pullies?
Spinning it over it's max will not net you more power. Instead, it will just super heat the discharge air. It has to operate within its efficiency range. There's no "20% more spin = 20% more boost". You can even lose power by spinning the blower outside of its efficiency range.
What's more is that you also get to a point (because it's a positive displacement super charger) where the engine is capable of aspirating at RPM, more air than the displacement of the supercharger can supply at each revolution.
It's not like a centri blower where the boost effect is the product of speeding up air flow.
If your blower is a 2 liter blower for instance, each revolution will displace 2 liters of air so long as.... It within it's efficiency range and has no inlet restriction at all. If you really want to try and get more out of it: Open up the inlet tract. Unless your blower is the bottle neck, you should see boost numbers climb. I saw 3 lbs of boost with my twin screw by swapping inlet manifolds. As a side effect, it also lowered blower discharge temps enough to bump timing up just a little bit more (mopowa). I'm planning the removal of even more inlet restriction but it's in line behind a ton of other projects.
The moral of my long winded story might be to look and see what else might make this thing as efficient at moving air as possible before trying to over spin it. A larger blower is the REAL answer.