I think you went about it the wrong way but I do believe Jiraiya can take Itachi, It was Itachi who said they would all die if him and Kisame fought Jiraiya, same result with backup. Kisame wouldn't accept that if he didn't think it was true, Kisame and Itachi didn't truly trust each other, respected each other, but didn't completely trust each other. This was shown when they were first partnered up, Itachi has his sharingan constantly on because of Kisame.
Now, If we remember what Itachi said about Jiraiya, If they fought (with Kisame and/or with back up) they would all die or be seriously hurt. Jiraiya confirmed this when he fought and beat 3 Peins, got surprised attacked by a 4th Pein and had to face all 6, willed himself back to life and killed another one. All Pein paths are Kage levels and Jiraiya took out 4 of them while also gaining information on them.
Jiraiya is a wise person and knows his limits and he spied on Akatsuki for years so had info on some of the members. He knew about Itachi and Kisame being in it and he knew of their reputation. He was alive when Uchihas were still around as well. When he arrived to save Naruto and Sasuke, he said he'll kill Itachi and Kisame both. He wouldn't say that if he couldn't, he would be thinking about how it might be hard or how he couldn't take them alone and try to figure out how to escape with Naruto and Sasuke. After just hearing a few things about Pein, Jiraiya had doubts about his chances and prepared for the worst case.
Both are equal in Stats but Jiraiya is higher in experience and not below in wisdom and smarts.