I suspect it has more to do with the gene pool in a particular area than climate. Around our place (pretty darn dry)the black bears are black. Black enough that I was going to assault the one in my yard one night with a Loisville Slugger in the mistaken belief it was a black angus cow (after making a positive ID, I retreated and left it up to the dogs)! Along the divide, just to the east, most are black but a lot have the white "V" on the chest. When we lived up the North Thompson, it seemed like close to a third were brown. I'll have to ask Lay what he thinks. He's shot more bears than most of us have seen and over a large and varied geographic area so may have a theory.
On the high speed 257 Roberts. I've always thought it made little sense that a 6mm Remington could do 3150 or better with a 100 grain bullet while the 257 Roberts was limited to 3000 at best. Like wise, if I can push 120's to 3000 from my 6.5x55 ( and I can), it's logical to think the 257 would come close to the same. GD