Through a combination of DIY work and other people's favours I have an interchangeable airbox/sausage filter system (They both fit onto the same backplate). So for noise-sensitive venues (Combe and Goodwood) I run the airbox, and elsewhere I run with the sausage filter. The airbox makes a *big* difference to the amount of intake noise. I haven't got a cold air feed to it at present, just a cone filter on the end of the airbox. It all fits under the bonnet, just, but it took a lot of fiddling about.
The airbox is the same as the one in the pics on Dave Andrews' website, http://members.aol.com/_ht_a/kenginemod/kairbox.jpg
Back-to-back tests with/without this airbox on a ~200bhp Elise showed that the airbox cost 10-15bhp at the top end. It also slows down the throttle response making the engine much less of a joy to 'blip'.
Mike