Hi Ross

For most impeller pumps and centrifugal pumps it wouldn't matter at
all, because they are designed to operate within a wide range.

But there is a better way than clamping down the output feed line.
Install a T-Fitting in the output line and a line connected to the
T-Fitting as a return line to your sump, you can install a valve or
clamp this line to increase output from the feed line.
This method works well on all pumps, keeps heat buildup lower and
places less stress on the pump.

We have similar set-ups on all of our bottle filling equipment, except
instead of a manual valve it has a spring loaded ball valve that can
be set at various pressures. When the filling head solenoid opens,
you have the desired head pressure. When the filling head valve
closes, the spring loaded valve is forced open with the excess head
pressure and allows the product to recycle back to its own carboy
(sump). Some systems use a split solenoid so that when one side is
open the other side is closed, but you get unequal head pressure for a
split second as the solenoid switches, which can cause a splash of the
product, so these split solenoids are rarely used. In fact, most
small bottlers and repackagers use gravity feed rather than pump feed
to save costs on electric and equipment replacement costs.

TTUL
Gary