Never had any issue's with that driver.I'm using 11.6 now,also with no problems.
I have 2 5770's and 1 5870.Using guiminer with the phoenix phatk-kernel.
Did you compare hash rates with 11.5 vs. 11.6? I saw a significant decrease in hash power when I 'upgraded' to 11.6 on my 5870. From 440 MH/s down to 380 MH/s.
You were right I did see a decrease, as fas as I can tell it was because the over clocking settings under "Performance -> AMD Overdrive" set the additional power back to 0% as opposed to + 20% where it was and because the clocks were set down from what their max was.
Please remember to use this in conjunction with MSI Afterburner to fine tune your fan speeds.