Ive always avoided such a topic for this very reason. Fact nobody truly knows itachi full potential. Now i believe kishi weakened him the same way he weakened nagato when both were alive with their disabilities because they were too op. Now we have seen what a healthy nagato can really do therefore ive little doubt in my mind that we are about to see what a healthy itachi can do, BUT even if we do see what a healthy itachi can do nagato will still be stronger than him, but this will improve itachi's chances of winning if it comes to a 1 vs 1 situation and yes he does have a chance of winning.
Fact is ninjutsu and taijutsu against nagato isnt properly working and that in instead inst true. If you can catch nagato's of guard and use amaterasu, it will cause him significant harm before he can either absorb it or shinra tensei it. As we saw when amaterasu got him, he was regenerating, if it wasnt for edo thats nagato greatly damaged. Now alot of people would say nagato can sense amaterasu therefore he would be on guard, thats true but amaterasu being of the fastest attacks, nagato would have to have a pretty fast reflex to iether absorb it straight away or shinra tensei it the moment it hits.
Anyway itachi's main fighting style being genjutsu would cause him problems because ninjutsu and taijutsu arent effective and that in itself is a myth as ive explained eariler. Anyway genjutsu obviously cannot be countered and nobody is immune to it. We saw that jiraiya has used taijutsu and ninjutsu and they were ineffective but only when he used genjutsu was he able to kill 3 peins and if that was nagato, thats him dead and he even acknowledged this after the fight. That is what itachi has over nagato, his fighting style is different from other ninja's.
Youve also got the legendary weapons, which obviously cannot be absorbed, with that itachi can attack with the totsuka sword. People seem to forget that even though nagato was healthy due to him not being able to use his legs for some time is still very immobile/stationary and doesnt have great speed.
Nagato will always be stronger but ive little doubt in my mind that itachi who already has him chances of winning, will increase once we see his full potential.
EDIT; I also believe itachi has the Yasakani no Magatama legendary weapon