I received a reply from Scientific Anglers including this information:
"Please bear in mind that the difference in many tapers is only a few thousandths of an inch, so it can be difficult to discern with the eye."
They also offered to have me return the line so they could check it out if desired. I got a chance to take it outside and cast it, and it casts fine.
I took a look at the only other full-sinking lines I have; a Scientific Anglers "Uniform Sink" WF-6-S, which has very noticeable tapers and belly, and an Orvis 350 gr. "Depth Charge," which, like my new SA 5-wt, has no noticeable tapers or belly (but has a color change).
I guess I learned something about full-sinking lines.
Thanks for the input,