I have a GBTA that is now 2 GBTAs. I had it under a phoenix 14k 150 MH and it was very green and nicely bubbled. That fixture blew out a MH ballast and I replaced it with a 4x54 T5. Now they bubble less often, and the color is very washed out. It seems 4 T5s are enough to keep them alive, but not enough for them to really blossom. I am adding some LED supplementation soon, in hopes of recapturing their former glory. I know you asked for studies, but I hope some personal experiences will help as well.