Little follow up. I remapped all the controls when I went PS3 mode, didn't redetect devices on RBR. Now I installed Fanatec drivers back while keeping the Logitech ones as well (shouldn't matter though). FFB now works with Fanatec driver and PC mode. I still can't remap the Steering Axis, but there's no real need as it works. All my mappings were recalled from the time I used the Fanatec driver the last time. On PS3 mode you have limited number of buttons, so it was bit different there.
[HKEY_CURRENT_USER\System\CurrentControlSet\Control\MediaProperties\PrivateProperties\Joystick\OEM\VID_0EB7&PID_038E]
"OEMData"=hex:03,00,88,11,7c,00,00,00
Has now changed to
"OEMData"=hex:43,00,88,10,13,00,00,00
Last 4 bytes is amount of buttons, 0x7c = 124 which around what CSW supports in theory. 0x13 = 19 which matches to button amount I have in PS3 mode. Not sure what the first 4 bytes are, likely axis flags, but I didn't see recent information on my quick search. The whole OEMData entry matches to G25 one now, don't think it's a coincidence. Same reg location on HKLM still has the old data, but I think RBR only reads from HKCU. All the buttons (>19) still work on the wheel in RBR, so it's bit of a mystery right now though.
Edit: No point triple posting, so I'll just small edit here. Seems it's enough to just change the first byte, ie. "03" to "43"
Had DX 8.1 SDK installed, so looked a bit on dinput. Seems to be this flag which you need for RBR. Shouldn't be issue with other games as Logitech devices already have it set by default.
Code:
#define JOY_HWS_ISCARCTRL 0x00000040l /* joystick is a car controller */