Frag --
What kind of edge did you have on the plain blade. In general, I find that plain edges will beat serrations on smaller-diamater rope every time, provided the plain edge has been sharpened for maximum performance -- low-angle bevels, and not too high-grit finish. The harder the rope, the better serrations do, the softer the rope, the easier the plain edge wins.
On the other hand, if you're talking about a plain edge direct from the Benchmade factory, it will do very badly.