If you are billing under the same tax ID then you are considered one entity, and therefore you are either all participating, or all not participating. It isn't possible for the dr's to be in network but the optical part not (at least in my part of the country). We actually bill for a couple of groups like yours, where the dr's are one 'side' and the optical department is the other, but as far as insurance companies are concerned they are one group. If the group is par, then you have to honor the contractual adjustment.
Michele