I have a Mercruiser Bravo. The impeller is pretty "robust" ........ Having said that, I have never tried a flow test with one but I do know that low flow WILL cause overheats that are directly proportional to the amount of available flow, at a constant power setting. AND, if the flow is low enough, there will be an overheat at some higher power power setting where the flow is not enough to handle the higher power output.
It's just simple thermodynamics.
You didn't indicate what model you have. If the boat is a 1998 year model, the engine and drive package could be one of the following:
572BPBYC 1998
57FCPBYC 1998
57FCPBYD 1998
572BPLKD 1997
572BPLKR 1997
574BPLKD 1997
57FAPLKD 1997
57FCPLKD 1997
57FDPLKD 1997
Looking at the parts breakdown for the above models @
http://epc.brp.com/ it appears that the pump is the same for all models (I didn't check all the part numbers) I suppose you also know the engine and drive you have is a Volvo SX so part numbers can be cross referenced to Volvo Penta.
Also, if there's a smaller pump that could be used on that engine, it's possible that someone installed the wrong pump. But, understand that the correct pump for that engine uses a #3856039 impeller. That impeller and pump housing etc is used in ALL the engines (from 3.0L to 8.2L ) It's MORE than adequate to flow enough water for a big-block!
So if there isn't enough flow, it's likely due to a restriction someplace in the circuit.
Also, your raw water pickup (either thru hull[if installed] or drive pickup) might be plugged with debris/corrosion/Zebra mussels etc)
If you suspect blockage in the pick-up or hoses from it, try connecting a larger hose directly to the raw water pump and throw it over the side into the lake and see if the pump output is better. If it is, the hose(s) and/or pickup is/are plugged someplace.
Cheers,
Rick