The caliber means very little in comparison to alloy, velocity, bullet design and construction, etc. I'd rather use a properly set up 30 cal than a poorly done 45-70 cast bullet.
The guys trying to use hard bullets and round noses at reduced speeds might as well be shooting FMJ jacketed bullets, of course they don't perform well, and the problem isn't the caliber.
IMHO you'll have to make an effort to get a 45-70 to perform poorly on game. On the flip side you have to put in a concerted effort to get everything right with a 30 caliber.
