I have the D801 and have flashed Kit Kat via CM11 but had mobile data issues and overall performance wasn't nearly as good as stock. I'm currently on a stock based ROM, NRG, and the performance is great.
One thing I LOVED about running Kit Kat was the ability to finally use the Bluetooth LE (Smart) function for my heat rate monitor (using Polar Beat, but there are other apps as well).
The problem with the G2 is that the BLE function is available only with Android 4.3 or higher, since it comes with 4.2.2 installed, there's no way to take advantage of that until there's an update. That's one thing that I'm now waiting for with an official update (or at least a stock based ROM that's updated).