Nice comparison. Good to see somebody finally put a different prop on the same plane and made notes.
However, it should be noted that while the props were on the same plane, changing the length by 2" is likely to have a significant impact on performance. Seven knots is easily believable. Van's makes no recommendation on cutting the length when moving to a three bladed prop.
I spoke to multiple folks at Van's at Oshkosh a couple of years ago regarding 3 blade vs. 2 blade. The general consensus was that the 2 blade is faster. However, nobody had any hard data. In addition, there were two people I spoke to in the both that stated they believe the three blade is faster and climbs better...specifically the MT. Again, no hard data so support that opinion either.
Bottom line, right now on this board there is no hard data on the same plane being fitted/tested with multiple 2 and 3 bladed props and tested.