What would be different if everybody could pull 7 gs instead of 6? It's not like everyone else would still be pulling 6. Everyone would have a smaller radius, better rate, and higher drag at corner speed.
I don't have a dog in this hunt, but it would add to the advantage between planes that handle well at high speed versus planes that don't. In the OP's example between the Zero and the P40, if the P40 could pull 7 G's instead of 6 at speed, and the Zero's limited turning ability due to high speed was only letting it pull say 3 G's (number from backside for comparison only) the P40 would have significantly more advantage, right?
I don't think I'm a fan of the suggestion. In game we already can withstand G loads over time with no ill effects that would pretty much liquify a human being. I don't think making that more prevalent would be a good step toward realism.
Wiley.