If it helps you sleep at night get one.
I know i did a couple experiments on our 421 Serval with a stocker vs aftermarket impeller and noticed no temp decrease. That coupled with the fact that i change all our water pump parts (seal bearing,pump,washer ,clip) out about every 3 years, makes me stand by OEM.
If you are worried about temps, id look at a higher capacity radiator,,but honestly i found more heat reduction by going with a richer needle taper than anything else.