You reach a point of diminishing returns when going richer with jets. The carb can flow "X" amount of air only, so when you up the gas concentration in the mixture, it's getting too rich for the air flow & power is lost. That's why people put larger carbs on motors (Mikuni VM 22-26, etc.) which can benefit from more air flow. The trade-off in carbs comes when there's just too much air flow for the motor. At lower rpms it will stumble off of idle if there's too much carb when you hit it hard out of a corner, so you need more motor, then...
It's the "Whiffle Bird" effect, a little bird that flies in decreasing concentric circles, faster & faster, until it flies up its own butt & disappears in a puff of feathers, too much of a good thing.