Stated In Manga:
Itachi had hinted both there kekkei genkai would destroy the ninja world
Nagato: Used all the rinnegan powers with little effort and easily destroyed a whole village
Itachi: Thanks to shisui itachi has the most powerful sharingan genjutsu and was able to kill his entire clan without effort
How are they on the sages level?
Nagato 6 paths and shared vision would be hard to beat them
Itachi could easily place anyone in a genjutsu and at his accuracy at hand with weapon's he could kill anyone with planning
Nagato with the sages kekkei genkai he has the biggest pool his pa
Itachi susanoo would defend him or them from anyone
Nagato animal summoning would destroy half an army
Itachi fire and water styles can't be beat in battle
Nagato asura path with extra limbs could easily use the meteorite technique
Itachi amatersu is far more powerful then sasukes
Nagato mastery over all elements would make anyone look like a child's game
Itachi uses of his grows would make basic genjutsu dwarf in size to other Uchiha
Nagato+Itachi= Sages level in combat of offense and defense