I have used several t5 fixtures and brands over the years and in my opinion you get what you pay for. The reflector has a LOT to do with the light output. Otherwise you could just hang a bulb out there and be done with it. I do just that in my grow window
(Excuse the photo quality - it's from my iPhone.)
Over my tank and lighted grow area, I use straight tube fixtures - one is a 4-tube 48" and the other an 8-tube 48". I have tried a couple different brands and settled on the New Wave brand because of their superior German reflectors. I use GE tubes, also because of better light output. I have one 24" 2-tube aquarium fixture in the grow window due to space and I use Giesemann PowerChrome Midday 24" tubes because they are superior in light output and quality to anything else out there. Speaking of getting what you pay for, the Giesemann tubes are $20 each! I just purchased two fixtures and 10 tubes to supplement what I already have and the order was over $400.
I have a thread from over a year ago where I include lumen/foot-candle figures. I measured light output at various distances and used a high quality calibrated meter. Do a search for t5 or foot candles under the lighting thread.
Good luck in your quest. I spent lots of money settling on my setup and would be glad to help you keep costs down by not replicating my mistakes.