Itachi and Kisame weren't planning to fight against Jiraiya. They had the diversion for that purpose. It's likely Itachi (and Kisame even less) doesn't know that much about Jiraiya's actual battle capabilities. They may just know the rumours. Their objective wasn't to fight against one of the legendary sannin, but just get Naruto. And as we learned, they were in absolutely no hurry to get Naruto back then. So, from a tactical point of view, it would have been pure and simple foolishness to fight against Jiraiya. Perhaps they could have won, but the cost could have been high indeed. And to be honest, Naruto now a few years later still wouldn't pose a problem for Itachi, so in that sense I don't think so much changed. They can act when Jiraiya or anyone else bothersome isn't nearby and the time is ripe.