Upload & Sell: Off
The most popular large softbox for full figure beauty/fashion work is (probably) the Elinchrom 74" Octabank. Elinchrom claims it's output is within 1/3 f-stop from edge to edge. I don't own the big Elinchrom Octa, but I've rented it on occasion, and can verify that it's at least as good (even) as the specs claim.
I've got a (much less expensive) Photoflex 4x6 box, which measures within 1/2 stop from edge to edge, which is pretty good for a box that's so much longer than it is wide and uses a front-facing flash head.
Darton Drake seems like a very capable portraitist. His work is very impressive. But, he's not here to explain what he did or didn't tell you about Larson boxes, so I won't try to debate his views. (Larson seems to be one of the sponsors of his workshops, so I assume he knows a lot about their products.)
I don't know much about Larson - They seem to be a smaller company that sells direct to consumers rather than through dealers like Adorama or B&H. But, if you visit their web site, notice how often they use the word "even" in describing the light output of a softbox: http://larson-ent.com/soft_box.php There's nothing I could find on their site that suggests to me that their 4x6 box is designed to be any less even than boxes from the more well-known brands.