There are a few things you can do to make the S200 a bit more stable. With the cylinder tight measure the gap between its end face and the block, then find a shim to go in there, this will make the tube/block assembly much more rigid allowing you to do away with the barrel band completely.
Also make sure the flats on the cocking peg in the side of the cocking bolt are horizontal. If they are not this can put a burr on the back of the cocking arm as the bolt is lifted up and locked down causing it to stay back occasionally which causes a low power shot as it will rob energy from the hammer. Well worth doing as you can bet if it ever does it it will be on your best round ever!
And bin the plastic trigger, the Rowan one adds greatly to the predictability of the release, but the trigger sears will need work to get anything approaching a good trigger!
I have been working on one with my brother over the last couple of years, his is really quite good now!
Your shot count could maybe be improved a little too, how much preload do you have on the valve spring?