I've always found 1250 ISO usable when nreeded.
Here are 2 from last year with FW 1.107. Both @1250 and the second is @ 1/20s
L1005784 Post LUF.jpg
L1005830 Post.jpg
Chris what do you have your lowest shutter speed set to and what RAW converter did you use.
I use ACR 99.9999999% of my work just because I'm so use to it and find C1 not to my liking.