Far to small and you'll burst the ball in the barrel, I've found that far to big and the ball starts "flicking" left or right or up or down towards the end of its range (although I've not read anything about this elsewhere so it might be something else causing it). In something that operates from a closed breach like a cocker our pump exact match or slight underboring is good as it stops the ball rolling out of the barrel but since an SLG operates from an open bolt you won't have that problem even with a massive barrel.
Since SLG's are cocker thread if you find its flying off target towards the edge of your range borrow a smaller barrel off someone at whatever event you're at and see if it make any difference. Most markers are cocker thread nowadays so someone will have one for you to try.