Just wrapped up a year-long custom job on a 1980 KZ650 and took it on the freeway for the first time today. I replaced both sprockets with new ones per the manual specs. I have to admit, it feels like it's geared really low - I find myself hit 5th (final) gear when I'm on 45mph roads. Around here (Idaho) there are plenty of 80mph speed limit interstates and it really feels stressed at 70 even though it's still got plenty of power. Anybody feel the same way with these 650's?
1980 KZ650F1 - 9k miles (current cafe project, nearing completion)
1981 KZ305 CSR - 10k miles (didn't have the heart to customize this one, will stay original)
The rpms can vary somewhat depending upon the sprockets. I would check the number of teeth to see if yours are stock are what.
My 81 KZ650-CSR can do 70 mph all day long, and speeds runs up to 85 mph & above are no problem. I've had several occasions when the battery & ignition coils caused the motorcycle to run poorly at speed. low electrolyte once, then a bad battery & bad coils another time.