This thread is proof that many modified cars would be better off being tested under the IVA scheme as a Radically Altered Vehicle.
Noise tests for IVA are 99db at 3/4 max power.
plenty of other things it would benefit modified car users:
airbag new rules
bucket seats
exhausts
emissions
brakes
suspension
etc etc etc
not forgetting the vast majority of the OEM car features meet and exceed the IVA requirements.
anything with any chassis mods almost certainly should be IVA tested and anything else with a lot of mods should come under it too really.
eg most of the cossie conversions on this site and out there generally should be IVA'd if built in recent years.
The biggest advantage by far of IVA testing is that the rules are clear in black and white. No overzealous copper can successfully do you for an exhaust if it meets the IVA rules.
Also by being IVA'd you get around a lot of the vague rules about parts differing from manufacturer spec as the original OEM spec becomes irrelevant as its superceeded by the IVA test pass.
The way things are going, most moderately modified cars will be forced to end up having IVA's anyway as they bring more regulations in to MoT and give the MoT testers a easy way of reporting such cars.