FYI, this mod, if done propperly will give a slight increase in throttle response and induction noise.
Anything, and I mean ANYTHING that reduces friction in an engine will increase power. This is no exception as the airflow through the airbox is less restricted, therefore less firction, less power lost trying to suck air through an over complicated airbox. It is however very minimal and will only reflect the quality of the work carried out. Ie, if you leave big dirty burrs and sharp edges you will be spoiling the airflow and it doesn't take much to induce a significant amount of turbulance into the airflow.
Classic car modification 101. Even Vissard would approve.