I am the OP from the similar thread over on the towing forum. I also have ordered a longer shank - going from 10" to 14". This is a Husky hitch.
I posed this question over on that forum: The 2014 Ford F150 towing guide says nothing about the length of the hitch shank. How do we know what length of shank was used to determine their tow ratings. Perhaps they used a longer shank and if we go to a shorter shank maybe we actually increase the capability of the receiver?
I am still using a standard hitch shank provided by the manufacturer of my hitch. I do not consider it an "extension". It is supposed to be delivered today. I should get it installed in the next week and will go through the WD hitch adjustment procedure to determine if I need to make any changes. I will be towing 1000+ miles in August. I will post back on the towing forum thread my results as well.