-8 C is no problem. I have found that when you start approaching -30 C is where the waste of time factor kicks in, even with supposedly temperature insensitive powders.
Awhile back I had to find some 30-06 loads and send the specs to a custom loader in Australia so he could load my culling supply. There's limits to what you can fly with. I know the ADI numbers for the usual Hodgdon suspects so that part was taken care of. Trouble was we were in the middle of a cold snap with -30 to -40 C being the norm. Keeping a chronograph running required a box of handwarmers and a sense of humor. It helped that the rifle is question was a Cooper that has proven that it will shoot just about anything well enough; but I didn't know that yet,
The real eye opener was that at around -40 the only powder that would break the 2700 fps mark with 165s was Varget and the rest weren't even close. That showed to my satisfaction that not only do all powders I tried suck in the real cold; the buggers change places too. For what its worth, the 50.5 grain charge of Varget and various 160s and a 168s is good for 3000 plus in normal temps in my 24' Cooper and starts to wake up around 20 below and above. It's sort of became a standard " If a 30-06 won't shoot that it won't shoot anything load".