20% and 15 lbs is pretty good, just means your motor is USING the boost instead of traping it in the intake. Turn that bitch up son. Mine is 30% over on a 509 with BMF heads making 18lbs. We have run fast with an 8/71 let it eat. You can spin it 40 over you got a long way to go yet.. The difference between 30-40 is HUGE and the difference between 20 and 30 is even bigger.. I can go about 48 over on mine but I will probably have my hihelix on it before then...depends on the ci and 871 on a 540 is to small.
on a 454 it's fine.
i got one, i know.
871 on a 540 with m2bb2xcnc head and it won't make enough boost , only 15 lbs 20% over driven.
head flow also makes a differance also.