Ok just spent an hour covering the boot floor with carpet and it made no- difference what so ever. My only guess is that carpet will not do the trick and you will need proper sound deadening which is expensive.( also my car is very loud and boomy, did a run to the tip this morning and a full load of bin bags made very little difference to noise)
I reckon this stuff may work but its not difinante, http://www.incarexpress.co.uk/view_product.php?partno=DYN10425

It may be worth taking you car back and see if they can pack the exhaust differentley.

Wheel well

Boot floor

Finished


EDIT: video of before run just to show noise problems
http://www.filecrunch.com/file/~3qxw25