Update. Replaced the charcoal canister for the final fix. At this point in time I have had several years of trouble free operation.
Bottom line. For slow re-fueling think first of a block in the gas tank vapor lines. First place to look is the CV Vent solenoid as spiders and other insects like to build their homes there. If the CV Vent solenoid is good, then think clogged charcoal canister. NOTE: you should be able to blow through a good working charcoal canister. So in my case the fact I could not meant a bad canister and hence WHY the gas tank was not venting correctly during re-fueling.
I know this thread is two years old, but I have recently been having the same issue with my 2005 Mustang where the service station pump continually shuts off prematurely. The latest mechanic* I've been to said I would need a new charcoal vapor canister, but he said he was unable to find one anywhere online. May I ask how you found a replacement one?? I am in desperate need of one and it seems like they stopped making them and are rare. If you found a good parts guy, I would love to contact them!
*The first mechanic cost me $900, which included replacing the vent filter and getting a completely new gas tank only to have the issue continue to occur, where they then suggested I get *another* new gas tank to try again. Fed up, I went for a second opinion, and after running a gamut of tests, the second mechanic concluded it was the charcoal vapor canister.