I managed to fix his stability issues somewhat by using the (retarded) missiles from the lame 'stealth force' mode. Basically they can swing out to elongate the heel. There's a little tab that stops them folding beyond 90 degrees, but it's pretty easily dealt with with some clippers. Then it can fold all the way down quite easily. I also took his batteries out which helped a little...I'd be interested in seeing if it's possible to remove all the electronics to further reduce weight, although that looks to be a lot more trouble than it's worth.
In the long run the best mod may be to remove the grey 'cape' from his back altogether and just reattach it when in vehicle mode, all the tabs along the side should hold it in place nice and snug...but that's cheating, right?