We only unlock the update to the latest version once we know there are no major issues and we don't do new builds after that so if you update via the update menu of HikaShop you will always have the latest stable build.
We could provide a zip with all the modified files. However, there are several issues with that:
1. We also modify the database structure on each update and that needs to be done unless you will have lots of errors. If you just update the files, you would have to do that manually where if you just install the new package via the joomla installer, hikashop does that automatically.
2. For 1.5.2 we modified 100% of the files and for 1.5.3, we modified a big chunk of the files.
3. We would have to build a tool to generate that zip and that takes time
4. Our goal is to provide overrides and plugins systems so that you don't have to modify any core files of hikashop
5. If you modify views, emails, translations or CSS via the interfaces we provide in HikaShop or make plugins for it, these modifications are not overwritten when you update so you should be able to update easily. There is not much left to modify, except if you want to add big chunk of features in HikaShop for your own use.