I actually did exactly the same thing over a year ago. I needed to download Cynogen nightlies quite frequently which was not feasible at that time as I was on limited data. So I used to download the nigthly on my digitalocean droplet and calculate patch over their. Patch sizes were just around 12MBs. I would then download the patch file and patch the old nightly on my local system. Voila! phone upgraded by downloading just 12MBs! I did thought why Google can't do the same thing with Android applications.:)