I'm happy to tell you installation is now a complete breeze:
- No more USB startup disk problems.
- The brightness function keys work out-of-the-box.
- The trackpoint buttons work out-of-the-box.
- No more graphics glitches (mainly text rendering) in Unity.
It even seems that the default font sizes are better suited for a WQHD display, although I can't actually confirm anything has changed here compared to 14.10. In the end, the only things I did was setting the "Scale for menu and title bars" to 1,25 in the display settings and changing the "Page zoom" to 125% in Google Chrome.
In summary: Ubuntu 15.10 just works on the 3rd generation Lenovo X1 Carbon. No tweaking required really. I highly recommend this combination for those looking for a new Linux based laptop!