I was at my LDS today, and examined two different elbows for the second stage. One was a 90 degree, and one was about a 120 degree bend.
The 90 degree bend elbow had a stem internally, and the air not only had to turn 90 degrees, but it had to go through two holes in this stem in order to get to the second stage. The holes did not necessarily match up to the opening from the first stage either. This, to me, would be a non-starter to put in the pathway for the second stage. You would go from almost laminar flow in the hose through a series of turbulences, and I'm sure that this would really restrict air flow under low tank pressure, and high demand at depth.
The second, the 120 or so degree elbow, looked like it would function very well. There would potentially be some loss due to the change in direction of the air flow, but it did not have any interference internally other than the bend itself, and that looked to be slightly beveled to help the air to go where it's supposed to go, to the second stage.
'Thought I'd throw this into the mix here; not all these elbows function alike.
SeaRat