I think Kaptain K is correct on both points, but I find it hard to imagine how a closed sphere could avoid reflecting 1% of the energy back into the photosphere. How long would it take for the photosphere to heat from 2000 k to 7000k with 1% of the energy being returned to the photosphere? Forever perhaps?
The 99% needs to go somewhere. Laser beams powering space craft? million gigawatt radio transmitters? 1000 times the number of far infrared photons our Sun radiates, radiated from the outer surface of the dyson sphere?
Is a "late" class M star cooler than an average class M star? Neil