In Reply To:
I'm not an engineer nor do I play one in real or any make believe life. The air after hitting the front of a softride is able to reform into smoother layers than a double diamond frame. You get smoother air hitting your back wheel w/ the softride vs the frame downtube churning the air. I would argue that the softride would be a faster frame than the standard double diamond frame no matter what back wheel you toss on.
edit: I don't think the frame, no matter what it's style, makes as much difference as rider position.
good thing you don't play an engineer because you would be fired.
the seat tube is the only tube on a bike that can lower the drag of a frameset. frames don't "churn" the airflow, wheels churn the air flow because of their spinning. frames disrupt it. that is why wheels have a bigger effect on a bike's drag then a frame (and account for more time savings if you have good ones).
which wheel is more important of the two. the front because it is in the direct line of the flow. the rear is not as important because
it drafts behind the seat tube. take that seat tube away and you have created a second front wheel (or closer to a second front wheel). a disc in this position won't be as bad as a spoked wheel.
in the heirarchy of time you can save on a bike the biggest time savings come from your position,
then your wheels, then your frame. the softride exposes the rear wheel to the wind allowing it to "churn" more air and thus creating more drag.