I tow with an '11 w/ 3.55 gears. My trailer is about 5k empty, up to 6500 loaded. The Ecoboost does great -- every time I tow with it, I am impressed again. I'd expect the 3.55 wouldn't struggle w/ the trailer the OP mentioned. After all, the 'max tow' has 3.73, which is only a slight difference in gearing.
But you will run in to payload issues with an 8000 lb trailer unless you get the right one. Like someone else mentioned, the F150 has a huge range of payload depending on whether it has the 'max tow' or 'hd payload'; with the latter, it can get up to 3000 lbs payload, more or less.
3.55 indicates it probably doesn't have max tow. As such, I'd venture to guess the payload is 1400-1500 lbs in XLT trim. Probably not quite enough for an 8000 trailer, at least not if you're concerned about ratings. A max tow w/ a payload in the 1800 lb range might do it, though.