I flew United a few years back when I was around 312lbs/5'6" and I remember it was tight, but I did not need an extender. I'm flying Midwest Airlines in a couple weeks to visit my sister and dreading it a bit... I'm hoping to get down to around 300 before I leave. Anything I can lose the better!
I'm considering asking for an extender as I walk on the flight, just in case, although I am less than when I flew the last time and I chose Midwest Airlines because their plane has 18" seats. I may also consider upgrading to first class for the right price. We'll see!