Hello Mint Developpers !
I'm used to use debdelta to make dist-upgrade on my debian desktop, to reduce the bandwidth usage when upgrading. Debdelta recreate the packages locally and only download the differences. If you could configure it on yours servers to generates the differences in packages between to update pack, i think you could dramatically reduce your bandwidth !
Of course, some work will be needed to integrate it to your update manager.
The only difference, client sided, is to do "apt-get update && debdelta-upgrade (to recreate the package) && apt-get dist-upgrade (which will download only the package that aren't recreated.
I hope it help !