I think the issue here is weight, not surface area. Shredders are light while rafts are heavier. If you look at the Russian video, those guys are getting worked by flipping over face first, likely because their own body mass (and center of gravity) is becoming the main consideration once their vessel becomes vertical. They also seem to be somehow strapped to the top of their pontoons, with all of their body mass atop the pontoon as opposed to a shredder where your legs (and the associated mass of your legs) are inside the shredder and at the midline, or lower, of the shredders pontoons.
Why don't we start running water falls in little rafts and shredders with the same crew in each and do a little experiment? The Pepsi-Challenge of vertical boating...
Also, did you see those pfd pants those Russians were wearing? When is RMA going to start selling those?