If I'm not mistaken, the Ninja 300 and the cb500 are very similar on the top end although the Ninja 300 needs to do it with more revs.
The cb500 starts to run out of breath above 80-85 mph but the revs are much lower.
The CB series is a great bike. Many people here have other more powerful bikes and enjoy the 500 just as much. Me included.
I think you need to demo one as the performance difference is not all that much between it and the little Ninja.
As another poster mentioned, there is an updated 650 coming out from Kawasaki. Maybe you should wait. I owned a 2009 650 Versys. It's rated at 60 HP and would tromp my cb500F in any speed test. Plus it's extremely reliable.
The cb500 gets 60-70 MPG. My 650 Versys got 52-57 MPG even riding aggressively and maybe nearly 60 mellow riding.
I'm not trying to talk you out of a CB but if you're looking for more power than the Ninja 300 take a demo ride first.