MNG uses Pocahontas bituminous as well. I've noticed the load we've gotten this time smokes a bit more than the old one did, so there's definitely some variance.
One of the problems with mixing coal is that you can never really keep a consistent ratio. I've had that problem working at Conway, where you do the best you can, but sometimes you run into a slug of anthracite stove coal in the chute. The firebox is so big, and the antracite so hard to get burning, that you'll shovel half the way to Conway just trying to keep the pressure high enough to not dump the air. You'll end up cranking the blower during the runaround, and then not be able to put enough water in it all the way back to N. Conway when the anthracite finally catches. You'll then spend all during the layover trying to keep the whole bed from going out over the next hour and a half. Mixing keeps the complaints down, though, so I guess that's a plus.
Anthracite is hard on a firebox, too, since it burns a lot hotter. I say it's better to throw it in the stove if the boiler wasn't designed to burn it.