A few years ago when I visited Chas Windridge of Mogsport fame, he suggested that to get much better breathing I should remove the restrictor plate that's spot-welded about half way along the internal pipe of the main silencer. (This plate has an approx 1/2inch hole in it, so reducing the effective diameter of the exhaust pipe considerably).
The method he suggested sounded rather brutal - remove the silencer and then batter this restrictor plate with a long and hefty steel rod until it breaks free - but I tried it, and it worked! It made a big difference, too. The engine is much more willing to rev, and the exhaust note has a bit more of a burble. And all this for zero cost.