Hello there,

You shouldn't need to download 15GB of data, the patch itself is quite small. This download is only required if the reoptimisation process is being aborted or skipped. Please make sure you run the game using the Launcher, as this will properly complete the reoptimisation process.

If it's asking to download that amount of information then it's trying to over-write the entire game client, so it would be quicker to reinstall using the Cataclysm DVD, and then patch from there.