to me, the only real advantage of the T5 compared to the T12 is the amount of wattage that can be squeezed a a given area...
watts is a measurement of energy, 746 watts equals one horsepower, 0.0002928 watt equals one BTU, etc etc etc, a watt is a watt is a watt, regardless of bulb design
so if we have two bulbs that both use 40 watts of electricity and both produce the same amount of usable light then both are equally efficient based upon our criteria
but if one bulb converts more of the original 40watts into heat instead of usable light then its usable efficiency drops, this is why incandescent bulbs typically aren't used to grow plants, too much energy is converted into electromagnetic radiation wavelengths that are of no use to plants, this is why a 20 watt incandescent doesn't put out the same visible light a 20 watt fluorescent does, and yet each is putting out the same amount of energy, just in different wavelengths
but with the T5's being smaller and of higher wattage, you can squeeze more watts into a smaller footprint, here is where the advantage lies
now which puts out more heat... the T5 or the T12, well that i don't know because even though the T5 feels hotter it has less thermal mass being smaller in diameter... so you would have to put bot bulbs in equal sized environments and then measure the delta T...
so at the end of the day would 200 watts of T5 be any more light the 200 watts of T12, probably not ( assuming the same delta t) but would the concentration of that 200 watts be greater with the T5, then yes... but this also assumes the fixture's reflectors both being of equal efficiency... compare a great T12 fixture and an el-cheapo T5 with no reflector and any gains would be lost immediately
according to the law of conservation of energy if you start out with 40 watts then you can only end up with a finite amount of light at 100% efficiency, regardless of bulb design.... but it's how well that energy is converted into a useful form (spectra) and how well the energy is concentrated (watts per foot, reflector design, distance to bulb, PAR, PUR)
that is the true key
but i may be just blowing hot wind too
Av