The helm wastes a ton of its budget on block rating, so the SBV from the shoulders is similar to the avoidance on the helm; SBV is quite nice, which you know if you care about the 4pc bonus anyway

The block rating from the shoulders is just as poor as the hit rating on the XT shoulders, so while they're both unimpressively itemized, the former makes up for it with a nice set bonus. On the other hand, the Vezax helm is much nicer than the set helm so the set bonus barely makes the difference.
Basically:
4pc + Vezax helm = nice helm, not great shoulders
4pc + XT shoulders = not great helm, not great shoulders, bonus hit rating
4pc + Saronite legs = not great helm, not great shoulders, amazing pants
I think the helm and shoulders are both unsatisfactory enough that as soon as I get a decent 245+ piece for either slot, I'll abandon 4pc.
By the way I think this should be in the Gear Discussion forum, sending it there now.