The whole thing about the original air box is to keep the noise levels down, and also keep hot air out. By removing that, and fitting a K&N, it's going to be noisy. Also, you'll more than likely be loosing power, rather than gaining any, due to the heat given off by the engine being drawn into the K&N.
A better way is to fit a Panel Filter inside the original airbox, remove the 2 pipes inside the airbox to increase flow, and fir the pipes from a Vectra C Diesel. You'll get a couple of BHP, and the wife will be none the wiser.