It tapers down to be very thin behind the edge, you may be surprised but at the edge, it is comparable to any of the thinner Spydies like the Caly and the Stretch. I haven't done any tests like mindlessly cut cardboard as I dont do that with any knife when I get it but I dont find the thick spine to be a hindrance and the Southard has performed well for my uses so far.
I attempted to take a pic of the edges of my CalySB, Strecth zdp, and Southard but my camera sucks, so no go on that, but yeah, Southard's very thin at the edge. The bladeshape at the edge side with its bellying is quite similar to my beloved Stretch, which I find to prefer for my edc. If it helps you out, I've sliced up phonebook paper with ease hehe. I'm assuming John (JDavis) and perhaps Ankerson will eventually do a rope/carboard test on it also.
Yeah, I compared it to one of my Paras (which isn't the thinnest out there but reputed as a wicked slicer) and the Southard's thinner than it down the edge. Same goes for a Delica4 I had out.