I've used the sleep system for several years. I generally don't backpack with the whole system as it's just too big and heavy. In warmer temps (>40) I've backpacked with the heavy bag only and been warm.
I've used both the light and heavy bag together when car camping in temps around 25...
What are the weights of your different loadouts?
Also, how do you travel? Military travel only? If you want to also use your bag in the civilian world, and are concerned about standing out, I would suggest to keep it to about 32 liters. Anything bigger and my feeling is that you start to...
I run a Mystery Ranch NICE frame and had issues in the beginning with the belt slipping down off my waist under load. I added an OV Innovations MEAN pad and that fixed the problem. I have a curved back, and a pronounced curve in my lower spine, so a more robust lumbar pad works better for me...