I don't see the point in retconning G5 into past fights. If I retcon G5 into past fights, Crocodile, Lucci, Doffy, Katakuri, and the rest are getting their ass beat a hell of a lot quicker because even if you take out the gags, G5 is too overpowered for those characters. It would be no competition. Marineford would be different because Luffy in G5 would raise holy hell to save Ace. He would unleash toon force like no other on Marineford, can you imagine that shit. If done right, it would be off-the- fucking-chain crazy, and I would fucking enjoy it.
Hell, by the time he reached Kaido, Luffy would have complete mastery of G5, and the Kaido fight would not have lasted as long. If Oda was a dark writer, whoo boy, the absolute chaos and horror, G5 Luffy could do while laughing like a manic.
The introduction of G5 Luffy that early in the story, and Luffy and gang would have found the one piece years ago.