Alot. but the stock Eaton M62 blower is really inadequate for anything other than stock power. If you want more performance out of it, you can switch to an Eaton M90 or M112. But the true better way to do it is using a Centrifigal Supercharger which one Grand Prix guy proved by running a 12.4 1/4 mile. But... its all a matter of avoiding detonation. I think the stock engine can handle 15 lbs of boost easily, just that a roots type blower (like all the Eatons) get really ineffecient after about 4k rpms and heat the air alot and thus do detonation. And an intercooler is expensive, so depends on what you want to do.
ADTR 1999 Black Bonneville SE
2003.5 Yellow Mazdaspeed Protege
2003 Azure Blue Mach 1 Mustang