[Bushwacker]

You can do a compression test on a cold engine; the absolute pressure readings will just be a little lower than on a warm engine (make sure the throttle is open to get max pressure). I'd be inclined to do that first just to make sure that the basic powerhead is reasonably healthy. Don't worry about hooking up flushing attachment for the compression test if you're gonna change the WP impeller anyway. What you're really looking for, typically, is to see if one cylinder is a lot lower than the others, indicating a scored cylinder, stuck rings, bad head gasket, etc., so you'll be focused on the relative difference between cylinders rather than the absolute pressure readings. The less variation there is between cylinders the smoother the engine will run; hopefully nothing over 10% but ideally 5% or less.

And after you've changed the water pump impeller, make sure it's getting water anytime you crank the engine. After you drop the lower unit and pull off the old water pump, take it to a local dealer you buy the water pump kit from to see if they'll do a pressure and vacuum test on it for a nominal fee to verify that the driveshaft and propshaft seals are ok! Then you'll know all is well before you put it back together.