The creators have never really emphasized the fight between G-Max Pikachu and G-Max Charizard. They emphasize the fight between Pikachu v Charizard and Ash/Leon.I hope that won't happen. Why would the creators emphasize the fight between G-max Pikachu and G-max Charizard, if it were to be like your script?
Anyway, I think G-max Pikachu should beat G-max Charizard on his own. If the Charizard is weakened beforehand by Satoshi's other Pokémon, then for me the competition between the yellow mouse and the orange reptile will make no sense.
1) Ash has been struggling with G-Max Pikachu with quite a long time now and one of Pikachu's greatest asset is its speed which the G-Max ruins. Plus, Leon has far more experience with G-Max and we've seen him essentially dominate fights after G-Maxed against other Master Class Trainers. Wouldn't make sense for Ash and Pikachu to jump to a level playing field with Leon in this regard especially when Pikachu's greatest asset, its speed is taken away from it.
2) Ash's Pikachu has not really taken up the ace Spot on Ash's team like Greninja, Infernape or Charizard. Even now as strong as Pikachu is, it's never really shown to be a cut above the rest of his team and considering the fact that Ash would be battling the ace of the Strongest Trainer in the World, it would only make sense for Ash to have a similar Ace Pokemon that Stands out like that.
Finally, if Ash did beat Leon in a 1v1/ Pikachu versus Charizard, that would essentially make Pikachu the Next Leon/Charizard level OP status which I just don't see happening.
I think something like this will make Ash's win far more believable but more importantly, it still doesn't necessarily make him super OP even if he wins the PWC. If Ash's Pikachu solo'd Leon's Charizard, then that would mean the only Trainers who can Put up a fight against Ash's Pikachu (Which will always be with him), are trainers with Pokemon as strong as Leon's Charizard. But having Ash use strategy and skill to win the fight versus only Brute force/Power, his Pikachu will be a lot less OP and it would be a lot more easier to bring in Opponents for Ash for 1v1 and make the series on the whole feel more consistent.