The frequency is really dictated by the ballast design, many ballasts run at high frequency but some run at a lower frequency because they believe that it improves effeciency. The increased voltage really helps with efficiency only, i cant really see it improving spectrum because that is mostly dictated by the chemistry of the bulb. Though im not completely sure, got any readable material on this ?.
I have read that you do get increased light output of up to 10% with the same power draw from a typical bulb.
Regarding 120v vs 240v traditional ballasts, regardless of the voltage input, the ballast rectifys and modulates the input voltage to a standard reference. Theres no difference.