Dick,
in the first instance why not ask CC for their noise data, The car must have passed SVA noise test which I believe is a lot lower than 108.
If yours is over this then something is wrong with your can/exhaust, if you go down the path of modifying it do you not void any warranty claim thet you may have in the future.
Please keep us posted on this as the problem will apply to all us R400 new owners.