On my tank I have MH and T-5.
If I run just T-5 my tank stays at 78 and the fans don't even kick on.
When I run MH, in the course of 8hrs my tank climbs to 80, and this is w/ fans on.
2 degrees isn't huge, but I'm trying to keep from running a chiller, so every little bit helps.
This is just my exp w/ my tank/fixture.