A high-flow water pump (with metal impeller- very important! Stock plastic impeller pumps have been known to fail often. The impeller comes off the metal drive shaft) is never A bad idea.
So (lets say for argument's sake) the 'fancy' pump is only 25% higher flow; that is certainly better than stock flow, and is probably assuming a sterile flow path. If you have a 10% restriction in your flow path, your new stock pump is at -10% flow. With a +25% pump, you are at +15% with the same restriction.
Since I have worked in thermal spray for nearly 25 years now, I know the importance of cooling water to a plasma gun. You need either high flow rates or high surface area to reject heat. If you can't change one, change the other to get more capacity. Higher flow rate does have some value. But you can also increase area with a six cylinder radiator in your case.
Radiator should always be checked, and cleaned and/or replaced as needed.
Run only BMW coolant (the blue stuff) and distilled water. Tap water is your enemy! Its not much more than the green stuff and worth it.
Water Wetter from redline is not a bad idea, but not necessary if you keep on top of coolant changes and maintenance.
Also, most (if not all) M44 engines have a provision for an engine driven fan. If you are
really paranoid about cooling, fit a six cylinder radiator, shroud and engine driven clutch fan in addition to the electric one and forget about it.