A couple of points....
Firstly, the engines were not designed to be sound moderated and any change to exhaust design is generally going to be an expensive affair requiring CAA (UK aircraft) approval. The PFA have a silencing competition running but I must admit that I have not being paying too much attention to it, albeit that this should be a less expensive method of gaining approval to reduce the sound footprint of a permit aircraft.
Penalties include cost, extra weight and drag. An additional six pounds of weight equates to 1 horsepower lost in my smaller aeroplane.
Secondly, it is an unfortunate fact that NIMBYs do exist and that they have been known to complain about gliders and hot-air balloons! A couple of weeks back, I am reliably informed by Ludwig, someone even complained about low flying by the Red Arrows!
Stik - quiet Pitts