As a datapoint, the Blichman 15g false bottom is rated for 1gpm ~3.8lpm max. When I ran it at 2.5gpm with 0.039" milling gap I'd sometimes get stuck mashes. I've since throttled it back to 1.5gpm and haven't had a problem. It depends on your milling gap and false bottom, but I've gone as low as 1gpm and gotten pretty consistent temperatures through the grain bed. I use chugger SS pumps.
EDIT: But mine is a HERMS system.