We have a 2015 Expy EL with the ecoboost. Power wise I have full confidence it could pull the 9K+ weight just fine. The turbo is a beast and there is little to no percieved power loss at altitude. Our camper has a 6250 GVWR and it has absolutely no issue with power pulling it.
I agree with the others that it is the payload that is the issue. I think around 6-7K tops is about the limit that you would want to tow due to tongue weight.
As for the squishy feeling when towing, we had the same thing happen to us when we first towed with it. The one thing that made the most difference was to just inflate the tires to max PSI which was 50. We just finished a 5000 mile tour of the Pacific NW. The Expy did great.