Depends on the Supercharger Setup used, Power is relevant to Amount of Air Flow CFM, Not Boost(PSI). Hence on a smaller older blower you must overdrive the blower to get more CFM, while a larger or more efficient blower can push more air at the same boost levels.
That Said. Dyno Records Suggest 3PSI or from the SC14 you can see around 25-30WHP (Around 35HP at the Motor/Crank). 8 PSI Should give you around a 50% Increase on most of sitaight six engines.
For More Power You Must Look into the M90 and M122H Setups. The M122H is 650HP Capable, In Particular would be around three or four times that of an M62 or SC14. And 2.5x the CFM of a M90.
Also Note this Nice Little Excel Calculator Attached Below, Put Together By James in the UK.
