Answer: The limit is when the rear tyre lifts off the ground and bike control is sharply decreased. As noted above braking too hard with the current short wheelbase MotoGP bikes and the rear tyre lifts off the ground – even at speeds above 150mph. I’m guessing that even makes Kenny Roberts a bit squeamish.
Same thing for acceleration – what is the limit to MotoGP acceleration? Answer: The point at which the bike begins to wheelie and steering control is lost.
Both of these limits can be lifted by increasing the bike’s wheelbase; and not by a little bit.
Part of the reason a longer wheelbase works is it lowers the dynamic, or effective, center of gravity. The lower the center of gravity – the less weight transfer.
While a short wheelbase may help street bikes’ walking speed maneuverability, and let you park in smaller spaces, a longer wheelbase will allow MotoGP bikes to lap notably faster.
Of course if you make the wheelbase (or track width for cars) too large other problems show up. However, I don’t believe we are anywhere near that limit for racing bike wheelbases.