Lets' face it, if there were a huge advantage for distance then everyone but everyone going for distance competition casting would use one now wouldn't they As far as I'm aware they don't.
Maybe no one has thought about it enough??
Simple experiment I have just done....
I have some pole elastic with me (dont ask)...and tied it to a pencil. I held the pencil upright and pulled the elastic back..just enough to put a little tension on it..and let go. This is as close as I can get to scaling down a fly line. Anyway, the pole elastic shot forward at high speed.
I did the same with some 50 pound nylon..similar diameter and no matter how much tension I put on it, it did not go any where near as fast.
Thinking about an easier experimet, if you had two lines, tie the lines off one end, then stretch them out...then let go. I bet the stretchier line pings back further than the non strethcy one with the same amount of pull applied. I would state money on it infact....meaning greater line speed.
Back to Mikes point...if both lines where the same weight etc, the rod would never load more with one line or the other..so..a stretchy line, all be it only a little stretchy should "ping" forward faster with the same applied forward cast..and thus travel further due to line speed.
BUT..now I am thinking
..arrrghhh...the "ping" of the line would occur at the moment in time when the rod unloads and releases the "pulling" part of the cast...so what is to stop the "elastic" from unloading not only from the fly end but also the tip end..and thus actually killing the cast and removing line speed....
So..after all that thinking...it probably does mean a non stretch line should actually cast further as there is not force in the line pulling in opposing directions....
Geeez...I am going to stick to Thermal Dynamics of Umbilical coatings whilst under pressure and torsion forces...so much easier as there are formulas to work with..lol