Welcome to the forum. The only Mystery Ranch pack I've used and own so far is the Big Horn. I love that pack. I often hike and hunt alone so I tend to pack more in case of emergencies. I always carry enough stuff in case I have to spend a night out or an extra night. (cell phones don't work where I go) You stated that you might be afraid of filling up your pack with that much room. However, that room gives you more flexibility to adjust to the different weather or length trips.
It carries 45 pounds very comfortably. This is coming from someone who had back surgery on an L4 disc in 97. Back pain is the gift that keeps on giving. I have never had my back hurt carrying 40-45 pounds in it. The extra weight for the better suspension is worth it. When I put the pack on the weight just disappears. I'll have to report back after this weekend as I'm going on a trip that doesn't have any water so I'll be packing a little heavier than usual.
In short the Big Horn would be a good pack for what your' describing. My only regret is I didn't get one sooner. Plus if it doesn't work out I hear MR is good about returns. Yes it costs more and weighs more but it makes up for it in room and with a better suspension. You're going to need some weight in order to carry weight in comfort.
