Electric pumps frequently pump significantly less than their rated flow when pumping against even a moderate resistance (head). A 30/40/50 or more gpm pump may reduce to 30 gpm or even less when pumping against the head created by the reduction in hose ID represented by a T-Stat, hose plumbing or the coolant passages in a head gasket.
The best performing electric pump I have found (so far) is a Delta-Pag unit that I ran across at PRI a couple of years ago. It is a for real 60 gpm flow rate with a brushless variable speed motor that rises and falls in step with coolant temps. Click here for more info =>
Delta-PAG Pump and go to page 4