Itachi can beat the 6 paths (high difficulty)... What he can't beat is Nagato himself, or at least he has less chances to win than has Nagato agaist him... And, he would have much less chances agaist 6 paths AND Nagato in the battlefield at the same time:
Itachi VS 6 paths of pain: 55% Itachi wins 45% paths win
Itachi VS Nagato (alone): 40% Itachi wins 60% Nagato wins
Itachi VS Nagato + Paths: 30% Itachi 70% Nagato
So, statisticly wise, Nagato > Itachi > Paths... At least, that's my oppinion! U_U