Hi guys! This is Eric from KineticJS. Thought I'd chime in
@atomic, if GS is meeting your needs in terms of the awesome additional functionality that it provides, stick with it! The Kinetic.Tween class is a super simple, light weight tweening library that provides the bare essentials. If you just need basic tweening, the Kinetic.Tween class would probably meet your needs, but if you need more advanced controls, I definitely recommend GS over any other tweening library out there.
Also, can you elaborate a bit more on the Kinetic.Tween performance issue? my tests showed that the performance is on par with GS (it should be, especially since it's very simple)