I went to Six Mile Gap to do some fieldwork on a lichen trimline site (a way of reading longterm river levels for 1000+ years). At 11:00 am, the Northgate gage was at 5300 cfs, which was last reached on June 5, 2005. Of those running the river that day, about half of the big boats (16 footers) flipped at Narrow Falls (which at this stage is a river-wide reversing wave/hole situation). It was a weekend, and quite a few people tried it. We were roping swimmers and loose boats in at the takeout for a couple hours. Carnage.
My guess at this point is that Windy Hole has some big lateral/compression waves, but is not a flipper. Cowpie is mostly washed out, with some big hydraulics. Narrow Falls will really toss your salad, with big hydraulics and compression waves above the drop (which is much wider than at lower stages with a big pourover hole on the right) and possibly a big reversal over the rock fence below. Tootsie Roll (the steepest sustained rapid) has several keeper holes on the right, some of which will flip a full-size raft. Stovepipe is a blast down the middle, with pourover holes right and center that you don't see at lower flows.
There was a party of kayakers doing the shuttle when I started out on my work. Maybe they'll post a report.
I'd say class IV and V kayakers will really enjoy it. Big rafts and cats will have a battle, with a high probability of a flip and a long swim. No paddle rafts allowed at this stage. Duckies, forget it.
For raft/cat boaters, a wise tactic would be to do this with no less than 3-4 boats, and to have a competent rescue crew waiting at Six Mile Gap with a kayak or two, and a bunch of throw bags to catch the swimmers and loose boats. The takeout is in a side channel on the left. If you dump, get to the left bank: there's a trail along the river below the takeout.
There's also a trail from the take-out that scrambles upriver to overlook Stovepipe Rapid. Sending someone up to look for loose boats and swimmers, with a whistle and a comm (cheap walkie talkie) is not a bad idea, plus they can take a camera and get great pics of the boats thumping through Stovepipe. If the boaters haven't done the run, a whistle and a wave above the takeout is nice.
At this flow, for rafters, having a catch team at the takeout might make a big difference.