We've got a 2014 XLT w/max towing and 3.73 gears. We tow a 28' trailer that weighs about 6,500# loaded. We just got back from a 4ooo mile trip from the Pacific NW to Colorado and Utah and back. This was our first big trip with the EB and I have to say I am liking it more all the time. We pulled over passes in Colorado up to a little over 12,000 ft with no problem. On the way home coming over Stevens Pass in Washington state a guy with a Dodge Cummins pulling a medium sized fiver passed us just before the final climb to the summit. I was cruising at about 62 mph and he was probably going about 65. As the grade steepened his speed started to drop off. I had a huge smile on my face as we cruised past him still at 62 (with very little effort) and he was probable down to 55 or so with a bit of black smoke coming out the back.
As far as the gears go, I think the 3.55s would work just fine as long as you're not going over any big mountains. You can always just gear down a bit more.
The mileage has been a bit disappointing but probably to be expected. We average about 9 mpg towing as long as long as we aren't going over a lot of big hills. I think on mostly level at a moderate speed it would probably be about 10.
Hope this helps.
Dewey