Retina 5k Issues

Wondering if anyone else is having issues playing LoL on there Retina iMac. I have a 27inch 5k iMac and it plays best at 2560x1440 but when I try to set to default 5k it makes the mouse tiny, and it wont travel across the whole screen. Basically takes the same 2560x1440 screen size and stays within those parameters. The screen also shifts to the top right making it impossible to change the setting back. Ruining a perfectly good game. If I do play the 2560x1440 and then exit out, everything on my mac is blurry/pixelated. Just want to know if anyone else is experiencing the same issues, if so, if there a damn fix for this?