Yes Argess, another one of those variables I did not mention, great point. If you need another shim, then you are running too much nitro. That alone can cause you to run erratic or hot, and yes, that would be noticable on a high speed run.
The heat gun is not going to tell you that you need to add or remove a shim for the fuel you are running, only your eyes and ears coupled with learning the engine will tell you that.

How do you think us helicopter guys do it? We can't come in for a pit-stop and flash a temp gun on the head. We judge by listening for bog or sag or the crackle of detonation and make adjustments accordingly. Sometimes we have tuning changes to make and if they don't work, we do plug changes and if it still needs help after that, we have shim or fuel issues to make adjustments with. Not one single change we make was because the temp gun had a reading on it, it is all bysight and sound or intuitive "feel". Changes are made to the tune or otherwise to optimize the running condition of the engine, not the tempreture of the engine.