Alright guys,
after some months of ext4, I decided to go back to f2fs. But it doesn´t seem to work, and I don´t know whose (ROM, Kernel, Recovery, me?) fault it is:
I tried to convert data and cache as follows:
1. Flashed the stock 5.1 image to have a clean base
2. rooted and installed latest TWRP (2.8.6.1)
3. went to recovery, factory reset, then converted data and cache to f2fs via the advanced wipe menu
4. reboot recovery
5. back in recovery again, went to asvanced wipe and checked, data and cache was indeed f2fs
6. connected my Nexus 5 to my computer, copied ROM and Kernel to it and flashed them.
7. rebooted device, set the rom up
8. rebooted to recovery (after setting up everything)
9. in recovery I went to advanced wipe and then to cache and data, and tadaa, file system was stated as ext4.
Is this just a displaying problem in TWRP (data/cache are f2fs, but TWRP displays it wrong?) or did the converting process fail/was reverted? I tried several ROMS (CleanCore, Chroma, Optipop) and Kernels (CodeBlue and Uber), but the result was always the same.