Also IMO they wasted the 3'd PCI-E connector on each board, even 2 per board would have given plenty of overhead for significant overclock (with the lower voltage per chip these aren't going to overclock as high on average as an S5 anyway).
I suppose their thinking for a 3rd PCI-E connector is when / if you used (cheap) PSU's with say 18AWG cables, you'd be able to make it up with the third.
BTW - based on my research 18-gauge wire in a common PCI-E cabling setup should be good for 7-8 amps at 30degrees C (NEC code doesn't appear to go below 14 gauge, so I had to extrapolate from wire cross-circular area) - so yeah, it would be definitely marginal vs. the capasity of the connector. Still plenty to handle the under-200 watts per connector of a S5+ on a 2-connector setup though unless you get the thing to overclock a LOT. On the other hand, cool running cables are a Good Thing so the third connector doesn't hurt anything (except perhaps the price by a few bucks or so).
Keep in mind that the "3400 watts" is the usual Bitmain "AT THE WALL" spec, the actual draw on the connectors would be more like 3100 watts total or ballpark 14.5 amps per connector on a 2-connector setup per hash board, since they claim to use a Gold-rated power supply for that spec (but never really state which one they use, some are a little MORE efficient than others).