As so many of us have after-market exhausts & the question of noise levels and MOT'S often comes up.
The fact of the matter is that what you need to do if you are concerned that your bike won't pass an MOT is pop around to your local garage and ask him if in his opinion your bikes emitted DB levels or OK in his opinion. If he says they are ok then book an MOT there, if he says no mate they are to loud then drive off around the corner and ask the next one.
The reason that this is necessary is because the level of exceptable noise emitted is down to the discretion of the MOT tester, it's as simple as that!
OK Some facts first....
NO Police officer has a Decibel meter
NO MOT station has a Decibel meter
NO Main Ministry station has a Decibel meter
WHY? because...THERE IS NO LEGAL DECIBEL LIMIT !
The relevant MOT section says you must NOT have a leaking exhaust or exhaust boxes and it must be gas tight, but nothing about noise...that applies whatever exhaust you have fitted.