You have to define "drift" when doing a 360 degree turn on the yaw axis. Assuming you don't move the right stick, and only yaw 360 degree's, the GPS will maintain a "Position" that is within 2.5 meters horizontally, and 0.8 meters vertically. So, it will not maintain an exact position throughout the rotation-but will appear to drift-within the 2.5 meter "limit".
Doing a perfect 360 takes a lot of practice. I do them in Atti mode-but I have an FPV transmitter+monitor, and a bit of experience.
Another thing you can do is this; get the Phantom going on a straight line, flip the IOC lever to Course Lock, then apply yaw, and see if it spins 360 degree's while maintaining a forward course.
Also-I recommend that when you're able, to upgrade to an FPV system, and add the iOSD Mini system. This will tell you on the screen how many satellites you have. It's possible on the rotation, that you may start off with only 6, but then lose reception from1, and this may cause you to lose the stability of the GPS.