I have the Muskeg 7000 on the Duplex lite frame, I like it and it'll handle the heavy loads, it's handled in the 150lbs+ range no problem. The frame is comfortable for me, rides well, but I know several guys that don't like how the hip belt rides, they have trouble with it slipping on them. I don't personally have that experience unless it's a super heavy load, then I have to readjust every so often but I've had that experience with almost every pack I've used under extreme loads.
The bag is a great concept BUT the Xpac is noisier going through the brush than your typical Cordura bag, and when it get super cold the bag almost sounds like you're rustling through plastic Walmart sacks when you're digging through it. BUT it's virtually waterproof, pretty light, and much more durable than I expected. I have a lot of miles with that thing loaded from summer backpack fly fishing trips to late season backpack rifle hunts and I haven't been disappointed.
I've tried SG and MR frames under load, both are great, MR not quite as good but have hunted with a couple guys using the Sky Talus bags and they had absolutely no problem handling super heavy loads with their SG bag/frame combos. It's very much personal preference, right now I just prefer my Duplex Lite frame.