He probably would said, "more monsturous" than he remembers Tsunade, if her was refering to 5 year old, but I'm sure he was told of her strength and abilities.
He barely got the news that she became Hokage.
And another funny thing is that how strong she was already before getting the Yin Seal, and the fact that Tsunade doesn't apply as much chakra as Sakura into her strength since she mainly uses it too keep a young body and save the rest for healings and regenerations. If she was in her prime...
But yes, she should get something related to genjutsu and be more damn original, but Sage Mode? Wow dont joke, that actually takes alot of time. It would be a huge AssPull, even bigger than the ones that just happened. And the fact that there are only 3 places to learn SM and only 3 SM: Toad, Snake, Wood.