My personal opinion, for what that matters, I don't know, Is why they stack so many lamps in a single fixture. To me it looks like you're wasting the 2 center lamps.
I just bought 3 new T8 4 lamp 8' fixtures, they are mounted but I still in the process of rewiring a few things, I think these may provide light over a wider area.
The ceiling in my garage is like yours, 14' on one end and 12 on the other. The by the workbench it's 8'. The 8' area is where the new fixtures are going. The rest of the garage has 2 48" T12 at the end of each bay and a 48" fixture along the sides (shine down between the car) It gives me plenty of light.