No. Modify it, take it to an engineer; they will tell you if it is or is not compliant with ADR. If it's not, you can fix it up so it is, return, and get a certificate; certifying by an engineer that the car is compliant with ADR, thus rendering it legal. The car must then be taken to a weight station, weighed... blue slipped again, done. (last time i went through it anyways which was after a defect/rego was also out... the blue slip guy barely looked at the damn thing once he saw the certificate!)
To the best of my knowledge; Atmo BOV's are to do with NOISE emissions.... nothing else! But i can see how it would mess up mixtures also...