The movie is set in alternate timeline where Nappa and Vegeta were defeated before they could kill anyone, hence Piccolo being alive and the main characters still being on Earth so Goku likely had more time to train and improve on the Kaioken so I don't think he necessarily has to be stronger than he was on Namek where he had little time to do anything because he was constantly fighting foes.M4 Goku vs Final Zenkai Vegeta? You mean the Vegeta that fought Freeza? Well, going by the fact that Goku's limit was Kaiouken X100 (as opposed to X20 when he was BP 3M), his base should be far stronger than Freeza Saga Goku, who himself is stronger than that Vegeta.
I think the limit of the Kaiouken increases with regards to BP, it says something like that in the Daizenshuu:The movie is set in alternate timeline where Nappa and Vegeta were defeated before they could kill anyone, hence Piccolo being alive and the main characters still being on Earth so Goku likely had more time to train and improve on the Kaioken so I don't think he necessarily has to be stronger than he was on Namek where he had little time to do anything because he was constantly fighting foes.