From what I gather, i dont think a new bolt will really make too much noticeable difference, not until you get up to top level guns, and small increases in performance really matter.
As what for what a venturi bolt is, i think its just a difference type of outlet for the air (i think its got 3 ports so the air hits the ball more evenly - but dont quote me on that).
I look around sometimes for bolts and upgrades for my Piranha, but cant really find anything outside of some of the bigger US stores, but Tippman kit is sold in loads of places, should be easy to get parts for it.