First, I would recommend you try using .600 round balls in your rifle. That way you can use those thick patches, but still be able to run them down the barrel without using a mallet. If you insist on using the .610, use a thinner patch, but use an overpowder wad to protect the patch from burning, and to seal the gases behind the ball. That will also give you better velocity with less powder, so you can reduce the load, and reduce the amount of recoil you have to deal with shooting the gun at a match.
As to winter shooting, I have not found the need to add more than about 5 grains of powder to my " summer" load to get similar velocities in most guns. The concern about velocities dropping off in cold temperatures is valid, in target shooting, but it has little effect in the hunting fields. You are shooting a short range firearm, off the shoulder, with open sights, that you just cannot hold as closely to an aiming point as you could a receiver sight, or scope sight.