I would make sure to run the entire river before you race it. If you do it once you should be fine. If you don't paddle it at least once I think you might find yourself deep in some big hole. In terms of difficutly expect miles and miles of cold class IV.
T, I'm having trouble commiting because the steamboat rodeo is that weekend, and I hate paddling 80% of the Upper A. It's cold, and not technical at all. Plus to really do well I would have to use a downriver boat. I've never paddled a wavehopper (and wouldn't in the Rockwood), and I suck at paddling round bottom boats. If it were all creek boats I think I'd be all over it. It's still a consideration.
I would definetly do it if we just ran the Rockwood box, or better yet run 3rd gorge lime into the Rockwood box. You should propose that next year. Have the mountain bike race end at Purgatory Flats (across from the ski area), then the boaters race their boats down a mile and a half hike, put on, have two portages, race the class IV creek, and then run the big water of the Upper A. Oh yeah, and hike out of the Rockwood to the parking area. Now that would be a sick and manageable race. I've done that run twice this year, and racing it I think it would take about 4-5 hours, plus you get some creek and big water to test paddlers in both aspects. If one log where cut out it would make the second portage easy. As it is you have to hand boats down a suto cliff. Plus the video footage in there would blow people's mind. Super scenic with fun drops, and creek boat only....that would be an amazing race without all of the other sports.
Seriously, mention that to the race organizers. Screw the 18 miles of class IV wave trains. Who the hell wants to watch that?