Swarbrick do race cans that fit onto the stock chambers (their number is in the tuning section). I had a set of alloy ones but they also make carbon ones. They added around 2bhp up top and 1ft-lb of torque.

Also If you look in the Marketplace there are cans there that fit the stock chambers and give the same results. They also coem in 3 differnt lenths.
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If I have to take the carbs off once more...