My new 14" shank should be delivered tomorrow. I will install it in place of my current 10" shank, adding 4 inches. I will let you all know how it works out.
When I read the Ford 2014 F150 towing guide, I find nothing that indicates the length of the shank and it's effect on the tow rating. It seems that the general consensus is that a longer shank will reduce the tow rating. Who knows how the tow rating was determined? Perhaps it was determined using a longer shank and anything shorter than what they used would RAISE the towing ability of the receiver?
I will post after I have towed with the 4" longer shank. I will re adjust my WD hitch if needed and will be towing over 1,000 in mid/late august.