With the extra power in the Wii, instead of speeding up the zombies and giving them more health, they can do something even better. They can increase the number of guys they can throw at you. Even with the current controls on pro, it is pretty easy to hold off fairly large groups with just a pistol and fancy foot work. They could more than likely have double the number of smart zombies active. I say smart as the Dead Rising ones are dumb as bricks and the civilians are no better.
Edit: LLLUUUIIISSSSSS