KK2.1 Hardcase Steveis v1.17S2 Pro issues?

Alright, so I got a KK2.1 Hardcase and I put it on the HK x650f quad frame with AQ2810-750kv motors, HK BlueSeries 30A escs, 12x4.7 props. Running a 4s 4500. Getting around 15 mins hover.

On stock firmware it was horrible. After upgrade there was an enormous difference, almost hands free hover at times. I kept my MPU gyro settings at 1000(since my sensor max was going ~550) and ACC at 4g. It flies very nice for a bit and then starts drifting around. I have to land disarm, sometimes wait a bit and then its flies fine for a bit and starts drifting again. At times it becomes very difficult to maintain altitude and also land.
Anyone have any idea what could be the issue?