My understanding with my 05 Outback XT is that it has a towing capacity of 2700 lbs. An M416 should be fine. I do notice a 1-1.5 mpg diffference around town witht he RTT and the awning mounted on the roof rack. I get 23-24 around town in my 5 spd manual XT. I get 25-27 on the interstate...not sure what that will translate to with RTT or with pulling my M416...but I am guessing it will still be double of my 76 Bronco on 35"s with 5.5" of SL, a 5 L EFI engine and a NV4500 5 spd. For cruisng back highways, the bronco can see 14-15, but for interstate its 12-13+...I am hoping that getting rid of the MTs and going to some ATs will help a little with rolling resistance next year.
To the point, a 5 spd manual should serve you well as it is really a 50-50 split. My turbo makes amazing power, not sure how that would translate to climbing a steep pass with 1500 lbs of trailer and RTT in tow. I would guess that it would do well, though. I love my Subaru. But I will always be in love with my Bronco...even though she is a thirsty *****.