"...that do a much better job of blocking radiant IR than any gas at any reasonable pressure that you can come up with. Forget about the gas that your greenhouse is filled with because it will have a negligible effect compared to the pane of whatever you use to permit the energy to enter."

You could be right.
But without any glass, a roof surface can get to 190 F. And without any greenhouse gases, but with an enclosed space one could probably get to 190 F air temperature.

My question basically is what is the highest temperature that can be reached? [and not using reflectors or such things.]
I think 190 F would be fairly easy, but is there anyway you get 30-40 F added to this? Or even add 20 F? [11 C].

Also, suppose you made something that made highest air temperature possible at Earth surface.
If you put it at Venus distance from the Sun, how hot would it get? [say you put it in space- at Venus' L-1 {oh, and assuming it doesn't melt & evaporate}].