Saskatoon
Your, at least, half-way right. The 757 Production Freighter will carry 15 125x88 inch ULDs. The 757 Special Freighter (Boeing developed conversion) will carry 14 125x88 + 1 LD11 type ULD. Commonly referred to as a "half-pallet".
In other words, by attempting to move the L/R1 door forward they would have gained an additional half position.
To my knowledge, Boeing crunched the numbers when they did the SF conversion for DHL, and it was found not to be economically and technically sound to move the door forward. An added benefit of retaining the original doors is that you gain a nice little foyer, rather than on the PF where you step right into the cockpit. The PF cockpit is incredibly cramped; on the SF you can at least step out the back for a stretch.