size of shower drain pipe

We are putting in a new master bedroom and bath and I have a question about shower drain size. Our shower will be 3' 4" by 5' 9" and it will have 3 shower heads hooked up to a 3/4" thermostatic valve: one regular head, one handheld on a bar on the opposite wall, and a rain-type head on the long wall. The two regular heads are rated at 2.5 GPM but can go up to 5 GPM with the flow restrictor removed. The rain-type head will do 6 GPM at 45 PSI. The only time more than one shower would be going at any one time would be if one of us uses the regular shower head and the other uses the handheld. Is a 2" diameter drain pipe adequate? Our plumber has just roughed in a 2" drain but I am concerned since a few people have mentioned that in their spa-type showers they don't get adequate drainage (I don't know the details of their setup, but I would imagine they wouldn't be less than 2" since they are new).