Ive got a silenced de-cat and I didnt really notice much change in the noise, exhaust sounds a bit raspier thou, sounds like its blowing but its ok Ive had it checked. Doesnt give you a load of power, it revs more freely like people have said as the engine can chuck out its exhaust gases better without the honeycomb mesh of the cat restricting it. Think youd lose a bit of power at the top end, I had a flat spot at the top of my power graph, maybe its due to the backpressue being lost a bit, but Im not sure. Im sure a replacement ECU would sort this out thou!