Two quick things: 1. The sim is smart enough to scan package deltas if you give it an arbitrary folder so if you fed it the content from the Alpha, it most likely did properly update only what was needed. I don't think there was a cgl update which is what takes the majority of the space, so that is a great "hack" if you can do it.
2. There are some open quirks with the content downloader. For example, on Steam it froze and required a steam client restart and then sim restart to actually download. If you give it a custom folder for content the download is very slow, if you kill the sim process and restart it, and navigate to the folder again it will now download at full speed. It still will take many hours. The sim for me also CTDs if you turn your monitor off and then back on while downloading content, if you launch it again it should pick up where it left off.