I came accross http://www.bavarian-board.co.uk/forum_posts.asp?TID=39033&PN=1 - that one shortly after owning my first E60.

Yes it is controlled by an electronic valve and a vacuum operated diapraghm in the inner tail pipe.  Hmm thinking about it there must be a vacuum supply from the pump that supplies the brake servo as you can see http://www.realoem.com/bmw/showparts.do?model=NB32&mospid=47756&btnr=18_0533&hg=18&fg=20 - here that there is defo two hose connection on the electric valve, unless there is a vacuum source in the boot/fuel tank area?

Disable it by disconnecting the vacuum pipe and then take the butterfly valve out.  Although there is no listing for the butterfly valve as a seperate part http://www.realoem.com/bmw/showparts.do?model=NB32&mospid=47756&btnr=18_0533&hg=18&fg=20 - here.

Although you might upset the back pressure in the exhaust system though which might throw up the EML light.

I know what you mean, it is a bit quiet isn't?  There isn't a throaty V8 roar when, ahem, shall we say, when pressing on. Would a different airbox and filter make a difference and supply more of an induction roar?

Andrew