Neil,
If compression and octane are right and there's detonation (read your plugs even if there's no knocking), that's a sign of too much heat in the heads/combustion chamber. Also, and this is the bad way to find out, if you start burning pistons and valves, usually in one or two areas, that's a for sure sign. Lastly, when the engine is hot, but off, loosen the sender or plug in the head and if air hisses out, you've got steam. If coolant comes out, you're probably okay. If it sucks air in as coolant comes out, you'll have to purge the system again if it draws a lot. Just crack it, listen and quickly tighten it. I'd just put a second temp gauge on there for a while.
Yes, Water Wetter can help. If there's something wrong with the system, it's just a band-aid and a bad idea. But in a healthy system, it works most of the time. There's other stuff called 40 Below and it supposedly drops temps by 40 degrees. But using all this goo isn't going to fix a poorly functioning system. It just makes a good one run cooler. Sometimes.
I fought overheating on my streeter coupe and finally corrected it with an expansion tank, not to be confused with an overflow tank (catch can). It just purges air as you drive. But there's a lot of braided lines running all over and a plumbing nightmare. For our applications with a close to factory radiator and engine position, you shouldn't have to do this.