I think, you are better off upgrading the frame + motors. While APM 2.5 is good, it is a hell lot complexer. You won't get plug-n-play with APM like you can do with the KK2. Get a good frame like the Flamewheel clones , and you can use other's PIDs as a base for experimentation and tuning.
I crashed yesterday , into a tree and broke one of the arms... replacement's gonna take around 20 days to reach me
Completely my fault, letting it go too far