Motorcycle-USA just answered that question by putting the new, 636cc 2013 Kawasaki ZX-6R on the dyno next to the old, 599cc 2012 Kawasaki ZX-6R. The improvement is actually fairly impressive, and that's before you factor in ABS and traction control.
As you can see, the differences don't just come in marginal increases to peak torque and power, but throughout the engine's mid-range, beginning at 6,000rpm. It's between that and 12 or 13,000rpm where most riders will spend most of their time, even while going fast. Fatter curves there equal an easier time, less throttle lag and potentially even fewer shifts. Nicely done.