There are no ADR's that say things such as "big turbos are not permitted" and "no BOV's permitted"... the ADR's say stuff like "you are not permitted to alter the emissions of the car by alteration of exhaust and intake systems" or something similar.
In this way I think if you change to any non-standard items in the intake or exhaust part you are effectively changing the emissions. Turbos, BOV's, exhaust etc are all parts of the intake and exhaust and will change the emissions.... this is where it is grey...