The guys at Balance Community did some testing on the various soft shackle types, including this one (big overhand). Their tests showed an average strength slightly (a couple of percent) higher than the button knot method.
http://www.balancecommunity.com/slac...ackle-methods/
My issue with the "big overhand" technique is that the knot is ugly. The button knot when finished is a work of art. I'm willing to lose [theoretically] 2% of the strength on my shackle to have something that looks pretty.