Stats are nice and do provide some insight into quality/efficiency/etc., but I think what is lurking beneath the surface of all of this discussion around Shaf and former QBs is the following: the ability to make the big time throw at the crucial moments.
This is where Lum, Stambaugh, etc. excelled, where Colvin, Shaf, JB have not. This is not a stat that can easily be teased out because it is affected by the timing or moment of the game.
It is on this factor that has separated the play and ultimately game outcomes for the various QBs.