My handy Excel spread sheet tells me that with factory gearing, my KDX200 would do 154mph at 6240RPM. I chose that RPM because it is listed on the steering stem, and I know my motor will actually rev to that level. Not saying that 7000RPM isn't possible, I just don't know for sure, if it is possible, it would put the bike at 178 mph. This calculation is with a 27.25" tall tire, and 13/49 gearing=3.77:1 ratio. Could be that your rear tire is slipping at those high speeds, keeping the revs high, but the bike only around 50mph or so?
I'll list the gear ratio's from Kawi's website:
1st: 2.63
2nd: 2.00
3rd: 1.53
4th: 1.24
5th: 1.04
6th: 0.87
Just a reminder, top speed is a function of horsepower, windresistance can double or even become exponential at high speed. Our KDX's may not be able to do the theoretical top speeds that I listed.