The fact that when Kabuto was pairing up Edos into teams who were near on the same level of each other already tells us who would win this. He paired Itachi up w/ Nagato. :/ That's considered Itachi's equal in this manga. (Healthy Nagato obliterates all though, bar Madara and Hashirama)
Itachi as a 14 year old defeated Jiraiya's equal (and I would even go as far as to say that Orochimaru is superior to him, he took on both Tsunade and Jiraiya w/ minimal help from Kabuto when his arms were sealed) in a matter of seconds, even when Orochimaru was the one who initiated the fight. Itachi didn't even have to look at him. He just activated his 3-tomoe sharingan and Oro was stuck in his paralysis jutsu. Orochimaru even has intel on Itachi and stated that he believed it would be impossible to take over his body because he is so strong. But Orochimaru believes that Jiraiya is weaker than himself. :/
Zetsu was confirmed to give the Akatsuki intel on various shinobi. That means that Itachi and all the other members have intel on Jiraiya. That's not helping Jiraiya's side, especially when he knows nearly nothing about Itachi.
In pt 1, it was pretty much confirmed that Itachi only made that comment about Jiraiya being stronger just to get out of Konoha as quickly as possible. He wasn't there for Naruto, only so that Danzo would hear about his appearance and remember not to lay a finger on Sasuke. Itachi's comment was ridiculous. "Even if we both go against him, and call back-up, we'll still lose no matter what." So if Itachi called Nagato and all the other Akatsuki members, then Jiraiya would beat them all singlehandedly? No. When Kisame met Jiriaiya, he essentially told him that he was disappointed. Jiraiya didn't live up to his hype at all. Jiraiya whipped out his most powerful jutsu, the frog stomach right off the bat and told them that they were toast. What happened? Itachi casually broke them out w/ amaterasu. Jiraiya was very surprised and didn't even know what it was. It took him 4 minutes to seal it with the scrolls. If Itachi were serious at that moment, he would have just turned around and looked at Jiraiya who was standing 8 feet away from him and completely covered him in flames. Jiraiya would cease to exist today if that happened. Itachi is overkill for Jiraiya. Itachi and Kisame are way way way overkill for Jiraiya.
Jiraiya is the one who taught Naruto how to break genjutsu during the 2.5 year time skip. Guess what happened when Naruto encountered Itachi right after the 2.5 year timeskip? Itachi just pointed at him and cast him in a genjutsu, w/o even using his sharingan. Then, when he tried to use the technique Jiraiya taught him to break it, it didn't work. Jiraiya isn't the best at breaking genjutsus by any means, it's even one of his main weaknesses. He has to rely on the toads, but what happens if Itachi genjutsus the toads?
What happens if Itachi amaterasus his hands? Amaterasu forms immediately and is spawned upon the target. How is Jiraiya going to seal it away if he has it on his hands?
How is Jiraiya going to hit Itachi, when Itachi is stated to be faster than him in the databooks? In fact, Itachi is better than Jiraiya in every category, excluding strength and stamina. Itachi's stamina is crappy due to his illness, but his fights usually go very quickly, so that doesn't ever matter. The longest fight he had was w/ Kabuto and that lasted 14 minutes minus dialogue and flashbacks. Itachi's strength score is actually higher when you factor his susanno and compare it to Jiraiya's incomplete sage mode. (Databook gives characters stats in their base mode.)
How is Jiraiya going to capture Itachi in the frog song genjutsu, when that requires a ton of prep time and he would initially have to hide from Itachi's view? Itachi isn't going to let that happen, he always monitors and analyzes his opponent and anticipates their next move.
How is Jiraiya going to capture Itachi in a swamp jutsu, when Itachi could most likely use his crow summons' and stay out of range or even move around in the area they are fighting to avoid it?
Jiraiya looked directly into Itachi's eyes several times during their encounter in pt1. How would Jiraiya get out of Tsukiyomi, when no one has ever broken it on their own and Itachi stated that it is impossible to break and that the only person to have a fraction of a chance at breaking it is an Uchiha w/ MS sharingan? (Obito implied that Itachi let Sasuke break his Tsukiyomi.)
Jiraiya is cool and all, but he is in no way beating Itachi. At worst, Itachi is taking Jiraiya med-diff.
