Yes u need to sight in for cold weather. There was someone on here that missed an elk because his tc omega was sighted in during warm weather and then shot in cold weather and missed. Plus his stock i guess was flimsy and moved around. I cant remember your name so if u want to correct me if i misunderstood go right ahead. As for you wanting a smaller sight setup thats easy. Email winchester muzzleloading and tell them what you have and that you want the steel sights that come in the winchester x150.
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Powerbelts are supposed to fit snug and the only problem i can think about with your gun, is that you are using pellets and not loose powder. All the cva's i own" 6 of them" Ive never had accuracy problems. Do not try 150 grains with the powerbelts, try switching to loose pyrodex RS and resight in with your 100 grain load. The max charge i would use with the 245 powerbelt is 120, and thats mainly because i use that load and its performing great. And to answer your floating stock question, i found on my rifle i had to sand out some of the paint on the side of the stock, when they dip it, all that stuff builds up and prevents the barrel from floating, run a sheet of paper down between the stock and barrel and find where your tight spots are and sand them down. I had to use a file to get down the thick areas. Maybe 45 minutes of slow careful work and its perfect. Make sure you dont take any material off where there is supposed to be contact at the "breech" area. Just take your time and slowly work and refit the barrel. But i think its the cold weather and the pellets giving u problems. More likely the cold.