I wonder why setting i.e. at least "Only First RAW in Series" does not have any influence in HiSpeed-Burst mode?
Probably because the Canon code in HiSpeed_Burst mode does not follow the same path as it does when the camera is doing normal shooting and so it is not intercepted by the CHDK capt_seq.c code.
Disable RAW @ Burst Enable [ ] / Disable [X]
result in RAW images in HiSpeed BURST mode?
No - disabling will tell CHDK not to disable saving RAW images. However, if RAW images are not being saved in the first place (see my comment above) then it really doesn't matter how you set this value.
Or did somebody find out/validate that this was too much for the DIGIC 5?
Not likely. As per my first comment, it's more an issue of the CHDK port not intercepting the shooting sequence at all the right places.
Bear in mind that saving RAW images to the SD card takes a lot longer than saving a JPG. This makes HiSpeed-Burst not practical in the first place.
Incidentally, the SX50 has built-in support for RAW in the standard Canon firmware. Have you experimented with that?