I used to live about 3 hours from the Upper Klamath and we never went up to run it in the winter. It gets pretty ugly up on the Modoc Plateau this time of year. We pretty much treated that run as a summertime option and given the alternatives, didn't do it too often then either.
I remember the run at 1 generator approx 1500 cfs, as good fairly continuous class 3 with 2 long class 4 rapids, Caldera, and Hell's Corner. I haven't done it at the higer flow, but I understand 2 generators, about 3000 cfs get's pretty exciting. I haven't been up there since around 1992 though, so my memory might be a little sketchy.