A fan clutch will only take away from power if the engine needs to cool. Otherwise it's just more or less free spinning. You could say your flex fan is actually taking away more power at lower RPMs because it's "working" regardless of temp.

Boy these tests all sound familar, as I too went through the same ones. At idle, a stock setup with AC going should have no problems. But what makes me wonder, why all of a sudden? I guess try the cheap and easy first (hose, seals, etc).

 

Thanks for helping me post my Photos.

 

Based of attached photos does anyone see anything obvious that would require attention. I have ordered lower spoiler and have seal material on hand for replacement.

 

Maybe I am the reading comprehension challenged, but I am still not clear on the readings. When you go out driving for a while, and the gauge goes to 260, WITH THE ENGINE RUNNING, have you checked the t-stat housing with the IR gun and the cap reading? What were they? If they were 260, then you do, in fact, have an overtemperature problem. If they are 200-220 or less, then you have a problem with your temperature gauge.

As others have said, when you shut the engine off, it is not uncommon to get some bubbling/boiling of coolant in your heads when the coolant flow stops. This is not necessarily bad, and given the high heat of vaporization of water, it should cool your heads down quite quickly. Also, I assume aluminum heads, with a higher thermal heat of conductivity than iron heads, are more likely to have this occur.

On your t-stat: Long-term, you want a t-stat, but you don't really need one in the short term. If it was me, and the above readings point to a gauge problem, I would go ahead and re-install one now. If the above points to an overheating problem, I would leave it out until the overheating problem is resolved, and then re-install it.

One other note: the only time I have had overtemperature problems in the two Corvettes I have owned is when I was running without a chin spoiler.