I would also inspect the area of the relief port (leading from the retracted valve position either to the LP side or to the pan). I suspect many aren't large enough to dump quickly especially with high viscosity and pressure. Some automotive pumps allow the piston to retract way past the port edge, meaning the port could be extended backward (examine the casting to be sure there's enough metal, duh). I wouldn't try to enlarge the circumference for exactly the reason mentioned: stability in the passage may be compromised.
In the Chevrolet L6 pump I'm working on now, the port is only 61% as large as the passage, the piston's OD is about 80% covered by the casting.

There has been some back & forth about why/when to dump excess back into the feed side as opposed to right into the pan: relief oil to the LP port causes turbulence at the inlet and may aerate flow? Not sure when this happens.