I talked to a really smart Aerospace PhD student (I'm a sophmore in aerospace), since I've been wanting a "scientific" answer for the aerodynamics of a twist barrel. He said that he believed that decreasing the turn rate of the spiral of the barrel wouldn't really affect the max fps of the barrel. I think it had something to do with the aerodynamics of a sphere, and something called the Karman vortex street. At a certain point, the velocity of the air will cause vortices to get thrown off unevenly, which will destabilize the bb.
http://en.wikipedia.org/wiki/K%C3%A1..._vortex_street
Although he wasn't 100%, and this isn't exactly his field, he's extremely smart, and I would side with him than just about anybody else.
Interesting video
YouTube - unstable velocity flow of air past cylinder with alternating direction
YouTube - karman vortex street
YouTube - Fluid Mechanics - Cool Science Experiment