Are group messages sent over iMessage when possible?
From what I have seen the minute a non iMessage user is added to a bulk message it sends all the messages via SMS. I suspect however this is dependant on the carrier more then apple. 

From testing, the verification of whether a user has iMessage enabled is done before the message is actually sent. 

My best advice is for you to try it out and see. I don't think there is really anyway to force it use iMessage.