I can't give an exact temp drop, but I can tell you I no longer use a chiller. :D I made the switch to T5 when my chiller quit actually. I had been thinking of trying T5 for a while, and I had been buying bits and pieces here and there for a couple months ahead of time when I found a good deal on a used ballast etc... Anyway, one day I was checking out the tank, and noticed the temp was up to 86, HMMM... that was strange, so I checked out the chiller, and the light was on to indicate it was cooling, but it wasn't! So... That day is when the halides came off and the T5's went on. After about 2 months I sold my halides, and I have not missed them at all! (Well except the shimmer lines.)
I think you will do well with T5, even overdriving they are cooler then the 2 250W halides I had on the tank before.