helicopters cant go over 200kts becase at that speed, the back rotors will loose lift. The only way to get past that is to us a counter-rotating coaxial pusher design like that of the sikorsky X2. Because the 2 rotors are rotating in oposite directions, the lift will never be lost. the X2 is suppose to be able to fly over 250kts, though its flight testing has just started and has only reached 106kts so far.
as for the sound barrier, helicopter rotors can easily go over mach 1.
a CH-47 chinook has rotor diameter of 60ft so its circumfrence is 188.4ft. if at 1000rpm, the blades' tips are turning at 188,400ft/min=11,304,000ft/hr
11,304,000/5280=2140mph. as you can see, its clearly above the speed of sound.