Hydro, no, watt for watt, T10s or 8s wouldn’t be more efficient with larger polished reflectors. T5s are more efficient because they are closer to a “point source” (geometric line) resulting in less restrike - there is simply less surface area of bulb to watt emitted. A reflector around a point source of light would lose no light to restrike, and if the the reflector were perfect, 100% would go in the intended direction.

I’m not an engineer and have very little technical understanding of these matters. I also lack a light meter–I hope to acquire one soon–but my subjective observations suggest that this fixture is a great deal more efficient than the T5 strip lights that it replaced. It looks to me like almost twice as much light, but the lamp is using just a little more than 1/2 the wattage of the lighting that it replaces.

I am also unable to comment on your point about reflector efficiency as it relates to lamp diameter, although I think that I get the general idea. If this relationship were to account in part for the greater efficiency of T5 lamps + polished reflectors, then perhaps the efficiencies of larger-diameter lamps, such as T10 or T8, could be enhanced with larger reflectors. However, reflectors thus designed would be very bulky.