I would check the function of the exhaust flap before I start replacing anything.
Recommend removing the tailpipe from just after the flap so that you can see it. Check that it is not stuck. Use the output tests to activate it and physically check that it is opening and closing.
They do like to get stuck, especially when the bike is only used for commuting and short distances. Corrosion on the shaft causes the shaft to bind and thus prevents the butterfly valve from opening and closing.