If you run a fan they can actually touch the light without problems, i usually give them 3inches so there is room to grow and adjust every couple days. they work awesome. if there fresh vegi watch them after a bit make sure there ok with it
i have to raise mine higher to cover more of the canopy there outgrowing the area after about 2 months.
I love my compact T5's for vegging. By running the lights so close to the top of the plant I get the short bushy plants I'm looking for. I also flowered under them. Towards the end I had ~400w of fluors in my flower cab. I switched to a 250w HPS and doubled my yield on the next harvest.
My air cooled HPS also added less heat to the cab since the combined heat of the all the fluor bulbs and ballast were in the cab.
In the end, I care about three things.
How many watts I'm am I running? Watts cost money.