He would be very strong but not Mewtwo Giovanni level. Agatha level seems fair enough.
If Paul was in M8 who would you be satisfied him loosing to? It's obvious anime team would not make him win vs Cynthia or Leon.
Eh, I think the anime more or less implied that Paul was on par with soon-to-be-Monarch Ash. He goes 1-2, but is fighting toe-to-toe with Ash.
He's using only Pokemon that happen to be same species as the aces of the other Masters 8, so not even his starter and possibly stronger Pokemon.
His Garchomp defeats Dragonite, which had beaten Iris' Haxorus earlier.
He's using borrowed strategies, not his own, that Ash may go up against, and helps Ash create strategies that he will later use. Ash even recognizes at the end that Paul intentionally battled in such a way to prepare Ash, and he was still that formidable. Paul isn't using a synergistic comp with entry hazards or statuses, which he normally employs to great effect.
The anime bends over backwards with the strangest reason ever to not include him in the PWC, because otherwise if he was said to have competed, that would mean he wasn't good enough to be in the Masters 8. He's almost like the final boss, Ash's greatest rival, who appears right before Ash's most important tournament and helps him enough to push him over that final hurdle. It also reaffirms Cynthia's foreshadowing at the end of DP in a somewhat different light. They may not have necessarily met in the Champions League, but it's still Paul who's there with Ash on the eve of the Masters 8, which Ash wins it all.
And I think it's the most respectful portrayal they could have done. Instead of making him Leon fodder like they did for Alain, the writers gave him a great ending. I think the fact that Paul is considering to be Gym Leader also leads us to infer that he did achieve his immediate goal after DP, which was to defeat Brandon. Knowing Paul, I don't he would have been satisfied for anything less than defeating Brandon with all three Regis.