T8 are a little over half as bright and consume around just over half the amount of watts as T5's when driven normally.
But T8 are just your standard florescent bulb, the T# denotes the size of the bulb and specifically the diameter mainly, but each one runs after different wattage's, but typically the smaller the bulb the more efficient it is.
the sizing is as such though T12 = twelve / eights of an inch aka 1.5", T8 = 1", T5 = 5/8" and so on
use to run floro tubes a ways back, they worked well for me, also had over driven T8 in my shop at the time, couldn't even tell it was night time till you stepped outside.
@Kalgrae you'll have more than enough light now lol probably pulling about 480-500w from the wall, could even flower under it if running new bulbs, that fixture will be pretty warm though, saying that the bulbs wont last as long, but free is free