Yes we do know that, they showed a little extract of the battle and we clearly saw that jiraiya was just trying to stop him and Oro didn't care about him and could even have killed him.
And what makes you think that Jiraiya fought to kill when trying to stop Oro ?
Itachi was on the good side, yeah but Kisame was he on the good side? I think not. He himself said that he wasn't on the same level.
Naruto at the time when Sasuke went away was weaker than him but after the Pain arc Zetsu clearly stated that Naruto was stronger.
That shows that people can improve. And Jman improved.
Do you think Oro could have done what Jiraiya did to Pain, he almost beat him. I highly doubt it
Do you think Hiruzen(old) could have done what Jiraiya did to Pain?
so he can only defeat orochimaru when hes fighting to kill? so he can't defeat him without use of deadly force then they are probably even at best.
no kisame isn't good but he did have trust in itachi and thought they were both on the same side, if he says they wouldn't win and they should run, they run, if he says hes only running because hes trying to avoid a big fight, kisame belives him.
yeah of course people improve, jiraiya wasn't the only sannin that got better either, oro didn't show that he had body shedding or edo tensei or his crazy giant snake summons untill current timeline so we don't know either of their "full" power when they went 1v1. Oro could survive alot of stuff, rockets/blades/chakra blasts from android pain no sweat, hes got plenty of chakra to fight powerful enemeies unless his host body is rejecting him making him weaker.
I bet he could kill several pains if not all of them, he was in akatsuki and knew more then anyone else thought he did (sasori's real form, got pain/itachi's/kazuku/deidara's dna) so if kabuto had nagato's dna to summon him but didn't have the chance to get it after he died then oro or kabuto found a way to get to him and knew his secret or it would have brought back one of the corpses he used for pathes instead of real nagato, thats one advantage jiraiya didn't have untill it was too late.
and 50-60 year old hiruzen im doubtful he could fight off all 6 pains alone because his chakra levels are so low he couldn't even afford to use shadow clones and couldn't finish sealing orochimaru's whole soul, hes smart enough to figure the linked eyesight and powers out but he'd need his young body and chakra to execute his plans.