Maximum frequency of a high-pass filter
In the limit, as the frequency approaches infinity the term involving C tends to zero. This makes calculating the magnitude easy, since the imaginary part vanishes. 
Mathematically, the magnitude of the transfer function is the magnitude of the top divided by the magnitude of the bottom. The top part is trivial. The bottom part is the square root of the sum of the imaginary part squared and the real part squared. Only the bottom varies with frequency. To maximise the magnitude, minimise the bottom part.