Hi Erik,
I have a GemII on a G11 and havn't had any probs slewing yet. I currantly only have a 12v 2a plug pack from JayCar at the moment but have ordered one of these
Powerwerx 30 Amp Desktop Switching Power Supply with Powerpoles
Has 3 outputs so can run other things at the same time.
I havn't had much paly time with the mount yet and it doesn't have much weight on it yet but have had it slewing aroung playing.