ORIGINAL: Mr Cox
The infrared gauge is not needed, it doesn't really tell you anything, the engine will tell you when it gets too hot by dropping in rpm. The tach is useful though and can tell you more accurately if the engine can hold the rpm or not. You should also have a clear rpm response when you change the main needle setting.
In his case a temperature guage is highly useful.
He is running a pusher configuration, where doing what you say is inapplicable.
If anything with a pusher engine the temp guage is practically the only way to tell if you have the engine tuned properly for what will happen in flight.
Been there, done that, learned it here.