It is usually said that if the exhaust gas is getting out quicker then you need to put more fuel in, and you will need a Power Commander or similar device to do this, I have never bothered to go this far, as changing the exhaust is normally just for looks and sound for me, on previous V11's I have tried ditching the airbox and fitting pod filters but it never seems to make it go any better, you will find that when you fit the Mistral x over it will be even louder, your bike will be spinning up a bit faster but maybe loosing torque due to lack of back pressure.