I did it in May, and there was current to within 5-6 miles of the takeout. The current moves very well the entire distance from 225 (Diamond) to Iceberg or thereabouts. If you have wind, it will be a chore getting across the lake. But I think it's absolutely worth it, even if the high cost of taking out at Diamond wasn't a factor. There's a lot of nice canyon and a few interesting rapids below there.
Some people do a night float after Separation, with the boats barged together and one or two folks staying up to steer away from the occasional sandbar. If you did that, you could be off in an additional day/day and a half. Also, be sure to take the required navigational lights. We budgeted three days, and did a more leisurely thing, with long stops at both Traventines, the Bridge dam site, and so forth.
There are lots of folks at http://groups.yahoo.com/group/gcpba/messages
who have been on the river since then, so you may want to post over there as well.
Have a good one.