I just added qty 4 dual bulb 4ft long T5 High Output Florescence to my 24x24 garage and combined with two LED's in my garage door opener I have nearly 40,000 lumens of light. It's a beautiful thing. Highly recommend them.
Here's two in front. I'll see if I can find more pics.
From what ive researched so far go with T8 fixtures. The Cost vs energy savings/added brightness of the T5's isn't worth it yet.
Id say that multiple two bulb fixtures are more beneficial that a pair of quad-bulb fixtures so you can place them around a car instead of down the center on the ceiling
I put 12 of these in my garage, 20 X 20. May sound like overkill, but worked out great, and am very happy with the setup. They're corded and would have preferred hard wired, but made them work. Attached directly to the ceiling rather than from the chains shown.