Well I'm not using T5 lighting, but I'll give some general replies, to help bump this up for others to see.
Do you have a link to a photo of these reflectors? The shape of a reflector has a huge bearing on whether its directly light into the tank, or back onto the bulb (restrike).
Any reflector you pay extra for should be made of polished specular aluminum. This stuff is amazing...it looks as shiny as a mirror, but is paper thin and light weight. the bulbs heat won't affect specular aluminium.
They will absorb heat, and will get as hot as the bulb gets, so I wouldn't recommend touching them. I'm not enough of a scientist to know how reflectors affect thermal dynamics...I would think with the increase in light beams directed back at the tank, you might expect a slight increase in temperature....but that's true of any reef tank with intense lighting. I run 2x96watt PC
's over my 40gallon, and as I understand T5 and PC
's get about the same temperature. in the summer I have to run a fan over my tank to keep it cool. Once the daytime outdoor temp drops below 85F, I can remove the fan.