I'd say it's probably somewhere in between the two, favoring the Gypsum guage a bit. It certainly isn't 1,500, but I doubt it's 650 either.
I ran it twice over the weekend in my 14' oar rig, first with 2 adults and 2 kids, second day with 3 adults and 4 kids. It was boney, but runable. Looks like it peaked around 1,800 on the lower guage on Sunday, but it certainly didn't feel lke it!
Expect to hang up on a few rocks here and there in the shallow areas. The noteable rapids are still fine (trestle, Interstate, Dead Cow and Rodeo), no problems there as long as you take the correct lines. We got a little right in Rodeo the second time and felt the rocks but did't get stuck. Trestle is a little more work to keep off the rocks (usually pretty much right down the middle).
I'd say if you are going to do it, you need to do it in the next day or so.