Its not the 3.5 ltr engine that effects your towing with the F150.
That little engine is about as close to diesel performance as you can get in a gas engine and with a lot less cost. What does effect the F150 is the frame and braking. Because I am at heart a very conservative person and like to enjoy driving safely; I would be very critical of any of the half tons towing a total weight of 11,000 lbs.
Now that might change with the 2015 F150 with reinforced frame and Alum. composite body. Its supposed to put the F150 close to Chevy and Dodge 2500's. If it does I think Ford will then use the composite body on the HD's to boost those numbers. Remember a lot of this is driven by the EPA for better gas numbers. Still waiting on the build sheets for the F150's. They should be on their web site by now but are not.