Nah, no way Tsunade would have beaten Pain man.
Eh, I don't think Kishimoto is consistent enough with power scaling to believe that. Just think about it, Pain was the strongest enemy we saw at that point, but Gai with 7 Gates should be able to beat him.
Tsunade fought 5 Susanoo-clad Madara clones simultaneously, she took a ton of punishment and managed to keep going. I actually think she'd fairly comfortably take Pain.