I think the reason for why he's abnormally powerful is the lingering question in light of the revelation that he's been having dreams about the Final Days even before he got his artificial echo.

It could just mean he inherited an exceptionally strong soul (The fact that he can perceive a soul's "color" was something noted as being rare even amongst the Ancients), but it's also suggested he may have been somehow tampered with by Emet Selch.