It'll also depend on how the paint fits in your barrel. If you're massively overboring then you're wasting air and will need to increase pressure/dwell to get up to velocity. I guess that would also be true for very extreme underboring, but at that point I'd be more worried about the paint busting in the barrel if there's enough drag to slow the ball down rather than create a tight seal and help it accelerate better, which underboring usually results in. Considering the stock PM8 barrel is a .690, and how small paint is now (I've had paint roll through a .679 insert)... that's very likely an issue you're experiencing.
This!! I'm looking to get my UL back bored out for freak inserts.