I haven't tested IMR7828 specifically, and haven't tested the SSC version at all. The IMR powders I have tested have shown anything from less than 2% velocity loss from 70 to 0 F., to around 5%, which is where point of impact sometimes starts changing at 100 yards--and that's the real problem, not 100 fps in velocity. If you rifle changes POI signficiantly with different powder charges when working up loads, then it just might change POI at lower temps as well.
When I am serious about making sure my rifle will maintain POI (and accuracy) in real cold, I use one of the Hodgdon Extreme powders. In the .300 Weatherby with 180's the Extreme to try would probably be H1000.
The Ramshot line is pretty good as well. TAC and Big Game seem to be about as good as the Extremes. Hunter and Magnum lose a little velocity in cold, but often not enough to change POI.
