Another interesting tidbit in that WSJ article:
In addition, research conducted by Boeing, airlines and others shows that the biggest comfort factor is whether the seat next to you is occupied. Studies show airlines would have to give four extra inches of legroom just to reach the same comfort scores created by an empty middle seat.
For me personally, I think the break-even would be closer to 5 or 6" as I
really hate sharing the armrest and unless my seatmate is on the small size, I am very uncomfortable in coach...