Maybe at the time of writing there is a better (at least easier :) way to fix the deep sleep issues, which might be introduced with a recent BIOS update — not quite sure. But anyway, here is what I did:

• Enter BIOS and go to Config — Power.
• There should be an option what operating system to optimize sleep for. You can choose between Windows and Linux.
• Choose Linux, exit BIOS with saved settings.

I left Thunderbolt assist mode disabled as well as all other settings in BIOS untouched.

Worked out great in my case, there is practically no battery loss during sleep state anymore!