Ran it at 800 several years ago and it is bony. some sandbars up top and rock dodging, more sandbars in Echo park. One boat got hung up several times in spots, just make sure you read the water well. Rapids are very easy at this level. I believe 800 is the minimum flow coming from Flaming gorge so you don't see it much lower than that. Rains had caused Red Creek to flash so we had very muddy water for our trip.
I think 1500-2500 would be awesome. Can't even imagine 4-5k and moving that fast due to the short mileage of the trip. All depends on water levels in the res and power reqs but you can pick up a lot of flow from the Yampa if you are lucky enough to get a permit during runoff. I still think low water trip is worth it for the scenery and hikes, you are at the mercy of the res though for the flows. The row through rainbow park kind of sucks a bit anyway, fortunately it's not too long.