A trailer has much more impact on windage than the weight. On ABRP you probably need to double or triple the energy consumption (wh/mi) depending on the size of the boat and how fast you drive. I have seen numbers from other members of 600 wh/mi and more when towing larger boats and trailers.
Regarding the load on the motor, the car experiences much more stress when you floor the accelerator to do 0-60 runs than pulling a trailer. I frequently tow a 4000 lb sailboat and a 3000 lb motorboat with my 20 year old F150 which has probably half the horsepower and torque of the Model S. When I pull the boat out of the water, the engine is barely revving. Therefore, I know that the load from pulling a boat out of the water is similar to mild acceleration on a flat road and nothing compared the acceleration of the vehicle at maximum power. Unless you are putting the pedal to the floor with a trailer on the back there shouldn't be an issue with load on the motor.
Also I have never had a problem with my F150 loosing traction with 4wd and rarely do the tires slip with 2wd when pulling the boat out. The Model S weights just as much as my F150 and I don't foresee any problems with traction either. Tesla also has excellent traction control which my F150 does not.
One thing to be concerned about possibly is braking as the extra trailer weight will extend braking distance even if the trailer has brakes. However the Model X I believe has the exact same brakes as the Model S and it is rated for 5000 lbs. I would definitely leave extra distance in front when towing to account for the increased braking distance.
Regarding mirror extenders, I never use mirror extenders on my F150 but we'll see when I try it out on my Model S.