I've just got 2 of them hanging up in my hood and the T5's are using retrofit reflectors (2 in front and 2 behind the Kessils).
My LFS has a Red Sea 150g set up with 8 T5's and 2 Kessils and it is absolutely beautiful. His corals grow so fast it's frustrating for me

. I'm running Blue Plus and Actinic in back and Blue Plus and Coral Plus in front. I really like the Coral Plus for great coloration. My LFS recommended both Blue Plus if going with only two supplemental lights, so that's what I started with a month or so ago. The T5's really add some brightness and color that LED's in general lose because they don't appear as bright to us (even though they'll burn your corals). This was my first experience with T5, and I've been told that ATI bulbs are the ones you need. The color and appearance of these Kessils over the Evergrow IT2040's I had is impressive. Both grow corals quite well, but I was getting tired of the weird disco ball color spots in certain spots.