What is the best way to show beamformer enhances signal of interest over interference
There's an article that defines some  measures for blind source separation that I used to measure the output of a beamformer Performance Measurement in Blind Audio Source Separation. Vincent E., Gribonval R. and Févotte C. Transactions on Audio, Speech, and Language Processing, Vol. 14, No. 4, July 2006. They basically use orthogonal projections to decompose the output signal and use these projections to calculate Signal to Noise, Interference, Distorsion and Artifacts ratio, I know the paper is based on BSS, but it could be an starting point.

http://www.irisa.fr/metiss/demonstrations-fr/bssperf.html