Two hours from posting my problem to getting a solution, I'm very grateful, and impressed
I guessed the issue was something along those lines, I've wrestled with CSS matrices myself before now.
Using parseTransform: true has fixed it for me, so I can stick with my current greensock version (don't want any more surprises when upgrading!).
Many thanks, and keep up the good work. As a well seasoned javascript app/game developer I can see you guys have a deep and throrough understanding of javascript/DOM/browsers and greensock is simply beyond compare in its field, it'll be standard on every project I work on in future.
Thanks again. Very grateful for your help.