You shouldn't have any issues as 3:15 trucks are rate 8,400lbs-8800lbs. With most other trucks it normal equals zapping the off-the-line movement but with Ecoboost to be honest the torque is so linear and outrageous it really shouldn't affect much at all... The EcoBeast has such bark off the bottom and the higher ratios only amplify that bark which again isn't really needed with 6k and prop even higher lol. Had you went with a traditional motor that has its peak torque up in the 4,000 rpm range you would notice having to rev high to get things moving... Your mpg will take go down some give it does no matter what load your pulling, but given the amount your pulling I really doubt it is going to be horribly bad nor will you be worried about not enough motor tomgetmthings rolling!
Milage will always be affected by towing heavy loads. I usually get between 20 and 24 mpg on the hwy. While towing my 9k camper I get right around 13mpg. I am extremely pleased with this milage towing, considering when I towed with my Tahoe I would be lucky to get 6-8mpg.