In RLB, I actually did notice a difference between an iron belt and glass belt. But then again, I did a lot of testing in the arena with some friends to see how much it affected their damage on me.
Though, I will admit that it needs to be a lower priority than completing his build. Making this upgrade should be after the other parts (armor and accessories) of your build are finished. Don't get me wrong, a GB is good. It's just that an INT priests endgame belt is a IB.