In the end, I 'solved' this problem by throwing money at it and buying myself a mid-2011 Mac Mini server (any excuse!).
After the initial slow sync (my home weighs in at around 45GB) subsequent syncs take a matter of seconds now instead of hours.What does tend to slow things down (to minutes rather than seconds) is if I swap between machines - for example, if I've been logged in as the mobile account on my MBA, log out then log in as the same account on my iMac.
Nevertheless, orders of magnitude improvement in performance so that the mobile accounts are now usable.
Perhaps my poor old mid-2007 Mac Mini just wasn't up to the task - though seeing as it's mostly just I/O, and it has a 1GB Ethernet port and a 7,200rpm 500GB disc in it (which I fitted, upgrading the original 5,400rpm 80GB HD) I'm quite surprised.
The other thing I've noticed that can slow down syncs significantly is if the server where the user's home directory is based is performing a time machine backup of itself at the time the client machine tries to sync.
I think this was a significant factor with my mid-2007 mini as it spent a considerable proporion of each day backing up (due to other backup related issues which I've since solved).