So, silly noob question here... LOL
Besides hearing the boost escape, what mechanical or performance benefit is there from a BOV?
There is a slight performance benefit. Stock BOV's leak under hard boost and can't keep up when adding boost. Aftermarket BOV's like this help to maintain boost in the intake tract so that your engine is getting all the boost that the turbos are making. But, getting this without upgrading the waste gate is in essence, silly. If your new BOV's don't leak your stock waste gate will definitely start leaking boost.
Keep in mind that not all BOV's on the market are high quality. Without knowing the design of this particular one, I can't comment on it, it does look high quality though. I personally recommend Turbosmart. I worked for them before and have handled them firsthand and know they are high quality and the company stands behind them 100%