I just took a 40 traveling, in swift no less, so no slouching or smooshing. I had it inside a larger zipped tote which fit pretty well under the seat in front of me, at least for take off and landing, because the way I had it positioned, I knew the handles were temporarily pushed to one side. But, just pull it back out a few inches for the rest of the trip and everything's fine.
I don't think I could put a 40 behind my knees since the base of the bag is so wide. Maybe a 35, but not a 40.
The story about the aborted take off took my breath away! I would have been terrified. Despite my miles, I'm a nervous flier. **shivers**