reporting back. First impressions of using the trackpad and mouse together are that technically it works fine and assuming you could chop and change between them when doing different tasks it would probably see off the pain from repetitive use of the magic mouse alone.
As far as combining the mouse and trackpad into a single workflow, photoshop works very well; pinch zoom and two finger drag to scroll with my left hand whilst using the mouse and regular tools with my right feels initially like it would actually be more efficient, seems to work best with inertia turned off. Unfortunately in illustrator pinch doesn’t seem to work.
I’m gonna keep the trackpad connected for a couple of weeks and see how I get on.