Yes, it finally worked! Did exactly as before but without formatting /system.
Soo... now I got a couple of questions that bug me:
1. How would I go about installing, for example, XROM or FXP 9.0.0 from this state?
2. Do I have to flash the stock rom, re-root etc. every time I want to flash FXP?
3. Can I test another kernel and flash the FXP 7.2.0 kernel back if it doesn't work?
4. Can I play with the Supercharger on FXP or is it rather for the stock rom and mods of the stock rom?
Thanks for reading.