The ones I have experience with, peak boost is at the highest rpm. It might make 12# at 6500rpm, but it sure isn't making 12#@ 3000rpm.if you have 8# at 5000rpm, what do you have a WOT at 2000 rpm. you should get 7 or 8 too. if you don't, then you are leaking boost, maybe your bypass valve.
Roots blowers should reach near full boost at all rpm. at least mine did