Agree 100%, they can be most evasive! That said I found Michelin to be quite open when, whilst their guest at Le Mans, I asked them regarding MO and MO1 labelled tyres.
Their answer reflects pretty much what you've said above, the MO and MO1 tyres were the best of the batch, in that they meet the much stricter QC requirements set down by Mercedes. The tyres themselves were constructed/manufactured no differently to the non- MO/MO1 tyres.