Bsieker :
As to the chances of avoiding traffic visually:
There is a phenomenon, [...] you tend to steer towards the object that you look at.
Indeed , one can even try this on a bicycle .And this remark is very relevant in the case of Ueberlingen. I always thought that should both a/c had been IMC they most probably had missed. Looking at the last seconds on the FDR of the TU154 could confirm this. But I speculate and this is outside the TCAS debate.
But being also a glider pilot, I can confirm that visual avoidance do work in the air most of the time ! ( even last minute visual avoidance, mostly by turning sharply away from intruder ).
But we are far slower and far more manoeuvrable than a passenger jet.