I had the resonator chopped off on my first 318iS, with hi-flo cat & stok muffler. It was not too loud, frankly I almost couldn't tell the difference. I also had a loud exhaust leak though, probably covering it up.
If you wanna do some custom fabbing lemme know. I have a MIG welder in my garage in San Jose. I am always down to concoct some sort of custom madness.