My settings and why:
Output - YCbCr
RGB - Limit
Super White - On
First I callibrated my tv with DVE BLU-RAY (video thus YCbCr output) so that blacker than black bar merged with black and whiter than white bar merged with white bar (as they are supposed to).
Then I went back to XMB and downloaded an RGB picture with BTB and WTW bars. When setting RGB to limited the BTB and WTW bars still merged correctly just like the blu-ray callibration. And when increasing brightness I could clearly distinguish the separate BTB and WTW bars, proving that even in limited RGB setting ALL GRAYSCALES were displayed!
When setting RGB to full, I got crushed blacks. Increasing brightness would resolve this, but that would mean that when watching movies this would now result in a too bright picture.
So grayscale in RGB LIMITED is comparable to grayscale in YCbCr, grayscale in RGB FULL is darker than that in YCbCr.