9.9/15s, especially salt water ones, have a chronic problem of a water tube restriction at the base of the power head. The design of the tube’s rubber grommet allows it to be able to close in as corrosion builds, cutting off water flow. The tell tale doesn’t really tell this tale, if you will. You have to pull the power head, remove the exhaust tuner that holds the water tube and clean out the corrosion. It is such a wide spread problem that I would not trust a single one of them to cool properly, unless I knew that service was recently done. Usually, the thermostat will be stuck OPEN, from having run hot so long. Not to be argumentative but I would like to say that I personally have never seen significant corrosion build up in the water jackets; although there is always a first time.

Like Bill says, the grommet problem is chronic on these engines. Is this a salt water engine? I surely don’t want you to rip the thing to pieces without exhausting all the simpler solutions first though. Was this engine running hot before the impeller change? You just changed the impeller, perhaps the liner/hsg/SS base are NG. Another chronic problem on salt water engines is the thin wall under the SS impeller plate. The thin aluminum erodes away under the SS impeller plate, allowing exhaust to be pulled into the water pump.
The overboard indicator stream is after the upper water tube grommet, so it would be affected by a crushed grommet. I have noticed a specific symptom on these telltales when the grommet is crushed though. When first started, the overboard streams seems "OK". Once the engine heats up, the stream weakens, then you will see a big puff of steam coming out of the telltale instead of water.
Tell us more about the engine, don’t want you to rip this thing to pieces for nothing.

Well, 190 is too hot, but I like that it cools down a bit when throttled up. This engine has a pressure controlled spring on the thermostat that allows it to be pushed off its seat with higher water pressure, which more or less bypasses the thermostat. You mention that your barrel water temp is 90 degrees which is pretty warm, so this might be the only issue. These engines like to get pretty warm when running slowly, which is a good thing, they run much hotter than older engines with no thermostat. Your temperature comes down a bit when throttle up which is a good indicator as well.
I would definitely take it for a run on a boat to re-evaluate. Post some pictures of the engine also so we can guess if it has been used in salt water. The engine sat for 20yrs, so hopefully the grommet is not an issue. You installed the complete kit, was it an OEM kit or aftermarket? Do you recall how the gearcase hsg looked under the SS impeller plate?

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.