I'm leaning towards John's earlier thoughts. I'm not so sure you're overheating.
It seems the assumption is a 300° hot section is excessive and although I've never measured one, I don't think it's out of line at all. That's not much higher than boiling water. In fact, the ABYC table of allowable materials for gasoline engine hot sections specifies by footnote that carbon steel (by far the most common on our exhaust systems) is allowable only for temperatures below 900°F!!! The point being, they're talking about temps in an entirely different ZIP code than 300°.
When we were trying to include exhaust backpressure as an
EWDS parameter we were looking for a pressure sensor that spec'd to at least 600°F. We found them but the vendors were awfully proud of them ($$$). With a little internet research, exhaust temps of 600° and considerably higher depending on mixture and RPM factors are common for gasoline engines
Your temp gun measurements seem in line everywhere else so maybe it's time to get a functioning temperature gauge before tearing things apart.
A side note about the hose routing
It's unusual in my experience for a water heater to be plumbed between the engine block and the manifold as yours is. You'll get more heat and better water heater performance if it's plumbed between the manifold outlet and the exhaust mixer. I incorrectly assumed yours was plumbed the conventional way.