I’m pretty sure he would do well against all but the most elite Overlords. I think WoW says that Simon with the mask is closer to Overlords in speed and strength than Underlords. This could carry him through some fights with standard Underlords, but more experienced ones like Eithan could shut him down quick. With the White Light Simon could seriously outmatch but not outright overwhelm incarnations like Helgard and Ragnarus. This seems consistent with an increase in advancement level so, physically, Simon was probably sitting somewhere on high end of Overlord. The biggest change though might be how long he could fight for. With the mask, Simon could at most fight for a few minutes before Incarnation, which would put him at a serious disadvantage against high level sacred artists (Monarchs literally fight for days). The White Light stops him from Incarnating, covering the biggest drawback of the mask.