Great info John . To summarize it " It isn't the size of Your drain, but how You use it " GOT IT !
Boston1 I wish I had more data for you on this. This info was shared to me by one of my drain suppliers as he was skirting the issue for why his drain was air locking. I went home and tried to see if the drain would lock but could not create the same environment.
I did find that the water level was higher in my clients home (where the 1 1/2" line angles off) vs my home's (my wife's laundry room aka mad lab) mock up.
Often the 1 1/2" lines we work on are in condo's - there we have sometimes 7" - 10" of free fall before the P-Trap. Never once a problem with flow rates here.
In the residential homes where we are building curbless showers often the free fall is 4" (2"x10" framing) and here you need to be very careful.
I have started measuring this as we build and then going back and measure water flow with a timer and 5 gallon bucket. Hopefully in time some kind of useful formula shows it's self.
In renovations that have multiple shower heads you really need to see how much water they can produce. When we pressure test the lines I ask my plumber to hook up two hose bibs and we simply fill a bucket. If the flow rate is in excess on 6-7 GPM then we send the water down the drain. Anything more than that should have a 2" line.
All the new fixtures have restrictors build in so most times it is a non issue. But rumour has it clients hate these and call the plumber back to rip them out.
JW