I'm late to the party, but decided to butt-in anyway. I've only got 6 MFS trips under my belt and they range from 2'8" to 7'8". The high water trip was in a GC dory. We had 13 boats that trip, all were very experienced boaters. None of us even considered sneaking, the hydraulics just didn't seem right. One of the guys was an old GC guide and his comment was that it was the biggest hole he ever ran on purpose. Most of the rafts got stuck and surfed right into the wall. We had a few smashed frames and a blown tube. I talked an extra passenger into my boat, took all of my beer and someone elses beer and put them up front. We ran the tongue into the hole dead center and popped through without a worry.
Up to about 5' the sneak is straight forward, above that I would go into it with a plan and not miss any oar strokes. Above 7' I would recommend bowing out and running another river. Up to 5', I also think running the falls is fairly straight forward as long as your boat is pointed downstream.
I see Velvet as something that can sneak up on you, but paying attention to where you are and looking for the rock will get you by. Of the rapids on the river, it seems to be one of the easier ones. There are plenty of places to get into trouble on the MFS. I had more problems at low flows with dodging rocks than moderate water. Personally, I think 4'-5' to be ideal.