It's highly doubtful that Jiraiya fought Pain with his full strength because 1) He had to conserve as much chakra as he possibly could because he knew absolutely nothing about Pain, while gathering information about him at the same time. - That's been stated that Jiraiya had to fight very carefully and conserve as much chakra as possible. 2) Pain or Yahiko + Nagato was his student. I don't think we've seen how truly powerful Jiraiya is. I have no doubt that he would have beaten Pain if he knew what he did when he died. Now, the main part I'm trying to get at is Itachi and Kisame. Jiraiya seemed so confident in defeating them and was actually pretty casual about it like, "oh, I guess I'll have to kill these guys now" and they actually HAD to run away from Jiraiya. The only way they were able to get away was because of Amaterasu. It seemed like he would have been able to defeat both Itachi and Kisame at the same time. What do you guys think? How strong do you think Jiraiya really was?