http://www.demon-twe...ng-exhaust-wrap
Ok, it may sound odd and like it wont work, but will only cost you a few quid to try if you dont believe it.
Wrap your hfc's with heat wrap tape, it completely alters the external tone, wrap up as far up the downpipe as you can reach also, make sure its wrapped tight and properly secured.
I passed the Thruxton noise test with hfc and true dual exhaust which is the strictest in the country, 90db and never blacked flagged on the drive by.