Many years ago, I tried to use a T10 bulb in a AllGlass light fixture (single tube) that was really designed to use T8 bulbs.
I had no problems with the light working, ballast worked fine with it too.
However, the plastic "reflector" part of the light touched and covered the top ~50% of the bulb. I did not have any way to measure the light output, but I would suspect I lost quite a bit since it could reflect off/back in to the tank.
When it came time to replace that bulb, it was somewhat difficult to remove it from the fixture because the plastic reflector warped.
My lesson learned (and my opinion) - using a T10 bulb in a fixture designed for a T8 should only be a temporary, short term solution.