It is important to keep in mind that on the 14.5" barrel, the dwell time is exactly the same as the 20" barreled rifle system, and on the 16" barrel carbine it is actually longer. This adds to the problem, as the maximum port pressure is much higer on the carbine length gas systems.
The critical pressure is the internal bolt pressure, which is a function of both port pressure and dwell...
One thing that is overlooked is the lock time of the rifle, on the carbine systems it is much shorter thatn that rifel and the weapon starts to unlock much sooner that it was designed to, or should anyway. Pigtails, U-shaped tubes and to a large extent expansion chamber designs all serve to "fool" the rifle into thinking that the gas port is located further out than it really is... the delay of pressure to the bolt caused by these systems does show some positive effect, but the down sides of them takes away from any measurable benefit.
As stated, on short barreled guns, the gas port must be opened considerably to get enough pressure to cycle the rifel, but bolt velocities are very high on these...