The new testing manual says “The exhaust system and silencer should be in such condition, or of such a type, that the noise emitted from the motorcycle isn’t clearly unreasonably above the level expected from a similar motorcycle with a standard silencer in average condition”. So it’s still a judgement call from the tester. I’d try it with the exhaust you have and if it does fail, you have a few days to sort it for a re-test.
https://www.mot-testing.service.gov.uk/documents/manuals/class12/Section-8-Exhaust-noise.html#section_8.1
