My iPhone 5 and iPad Air, both running iOS 8, have both been absolutely rock-solid with 5Ghz wireless since their upgrade.
I'd even say Wifi performance is better - I'm getting much less drops of signal as I move around my garden, for example, than I used to, it now doesn't drop at all (garden is about 40 yards long) whereas before if I walked to the top, I'd lose Wifi.
Note though I am using a combination of Apple Airport Extreme and Airport Express, if that has anything to do with it!