We have a 2" O/D exhaust all the way from the LCB manifold to the back. We have 3 Jetex straight through silencers to keep the noise at a comfortable level for road work, but it still gets a nice 'note' at 5,000+. At Goodwood last Monday we only just passed the static noise test with 97db, against their track day limit of 98db!
Maniflow suggest 2" only for 100bhp+, and caution that it will be noisier than their standard 'big bore' LCB, which is 1.625" OD.
This link will give you some idea about the noise level from Goodwood last Monday - my son Julian was driving, and he is quite gentle with his mum's car!! The last clip is more typical as the track had dried out.
http://www.youtube.com/watch?v=1SWeS0Hgvxw