isnt AMG a different car to Mercedes?
Anyway, if the MOT garages where told to measure the exhaust noise it means more time will be spent on a test and extra equipment will need to be purchased and calibrated each year. If this was done then the MOT fee would need to increase and no one would like that speacially the people who have not modified their exhaust.
The inspection manual that testers use are full of subjective and objective decisions. Subjective are decisions like excessive corrosion, play in a steering joint and exhaust noise. Objective decisions are things like 1.6mm minimum tread depth and 1.5mm brake lining material.
A tester has to use his experience, qualifications and knowledge of different vehicles for subjective decisions and if he thinks a exhaust is louder than that of a standard one in average condition tested at idle and 2500rpm to 3000rpm, he can fail it. Remember this is a static check and not under load.
The problem is that the testing personel i have experienced are not the sharpest tools in the shed. Motor mechanics is not an overly educated subject either. There should be one max noise level and thats it, measured by the operator if it appears to be ott. That won't increase mot times at all.