Interesting question!
Two things I do know about rear tapers:
1) it allows more length to be shot into a overhead false cast without introducing a hinge effect
2) if you let it overhang completely while spey casting it will kill your cast, in other words it looks like the back of the head but it really is not sufficient to turn a spey head over unless stripped into the guides or even to the hand sometimes (holding as mentioned)
Therefore we can assume that the more gradual the back taper, the smoother the turnover provided the working length of line is not exceeded. A zero back taper can be stripped right to the tip for spey casting but a long back taper requires a little inside the guides.
But your question is, can we assume that it helps the aerodynamics of the cast in flight? I don't think this is an easy question to answer. It has more weight than running line, but probably offers more rear stabilization.
I didn't know Ed uses a back taper for "holding line", very interesting. That's simlar to the idea we had on the coast for managing shooting heads with slickshooter running like, i.e.: the "redzone". The idea is to add 18-20 ft of a fat running line via blind splice to the slick shooter permanently. Then shooting heads are looped to this line, providing holding and overhang slip control for overhead casting. Sean already did it for his Atlantis running line and likes it a lot. I've been thinking about it a while but didn't try it yet, will do it shortly, maybe with the miracle braid because it makes such nice internalized loops (it's a braid already).
I know one thing, if manufacturers started to put long back tapers on their spey lines I would prefer the color change to occur where the operative length of line is found, not at the junction of the back taper and running line.
Interesting discussion, I hope some of the scientist types chime in
