So a temp gun is only going to measure the surface temp not the actual ambient temp. It is not as accurate as using a wired temp gauge with a probe. You would tie this in at the basking branch below the heat.
I do not think your really getting 80 with just the T5. Reason why I say this is I run 3 on my cage that is a hybrid with solid sides and get 80. So I would get the new temp gauge and see what your real temps are.
But the 100 watt is most definitely too strong. If it is a basking bulb with the flat top then it is going to produced more focused hotter temps as well. 60 watt regular incandescent is your best bet.