Thanks for all the input! To answer your questions, Timetraveler, I suppose I could remove the brake plate, but then I would have no brakes! Are you thinking its not getting enough cooling air?
As far as I know, my comet has the original mag and ATD, timing was checked and set with a degree wheel when head was off for the piston replacement, about 4 degrees btd fully retarded. ATD seems OK.
I don't think emmisivity is a factor here, if it was the actual temperature would be even higher, check out the reflection on the fender in front of the exhaust pipe. Dull, as in not shiny, surfaces are fairly consistent, camera was set for .9 emmisivity.
As far as Yamaha vs Comet, same engine size, same fuel (premium), similar consumption, mid to high 40 mpg, Comet bore/stroke 84/90 Yam, 87/84, more or less similar performance, Yamaha revs quicker (smaller flywheel) and is geared lower, top speed might be higher than 110 Kph, but I'll never find out, its pretty scary loose at that speed, Comet will probably do about 80, I have had it up to over 70 before it seized. AFAIC they are as closely matched in output and efficiency as any engines from different manufacturers and design could be.
I will take the advice of those who say wait and see, the comet only has about 25 miles on the new piston, I will check it again after I have a hundred or so miles more on the piston.