Dave
Sounds like the engine was used only briefly in salt water, and has low hours, so the grommet is probably not an issue. I am thinking the warm water temp in your barrel is probably the only issue here. Where are you checking engine temp? The exhaust manifold is water cooled and should run relatively cool at all times. Engine temp is usually checked right on top of the cylinder head, perhaps your pyrometer isn’t accurate. Again, I’m thinking things are OK, especially if engine temp comes down a bit when RPMs go up. Is there any evidence of overheating issues, such as melted wiring or discolored powerhead paint (if your powerhead is white)?
The other issue I mentioned is with the gearcase housing, not the SS impeller plate. And again, sounds like this is a low hour/fresh water engine, so I doubt the aluminum housing has eroded away causing an air leak under the water pump.