That is very close to Hillmann's Traffic Cop method which I use on all of my young dogs now. I believe it is the best way to instill the 'sit' command.
The only difference is that he mixes happy bumpers with the rigid obedience letting them have a 'recess' if you will. It teaches them to 'turn it on' and immediately be able to 'turn it off'. That is Bill's 'philosophy' for the day.
One more thought.......just my thinking though.......if your dog is not steady, you don't need to be working on quality marks, you need to be working on sit like Dave mentioned until he is. That is steady to an easy hand thrown bumper in the yard, then up the excitement gradually until you finally have a gun station, bird thrower and noise from the gun station. Anything less is asking for trouble!