I have a similar problem with my twin out for my 3.2. Except it isn't me that worries about it. I love the sound but my better half finds it unbearable at 2500-3000 rpm.
So I made one of these for the back. using some 2mm plate a spring and some split pins. It is closed till about 3500 and opens when and as required
Please dont hold it against me for using a brake piston pad for the cap. I know it is a bit rough, Surprisingly it works great, The only down side is the Motronic finds it hard to adjust. When I remove the cap it hunts and idles rough until it learns the new set up. ( for lack of better words) this is also a concern as I do not want to run it lean so I will be putting on a dyno soon to see the difference on and off.