In full disclosure, I have not tried any 777 with a 3-4-3 in the back (nor would I want to). The hard facts and maths leave me feeling skeptical about how 17 inches could even be remotely comfortable. As a mental exercise, I've thought about the reasons why I could be wrong:
1) A window seat could be tolerable given the extra shoulder width due to the curvature of the plane and the impossibility of someone's behind rubbing up against your shoulder as they walk up the aisle
2) Perhaps there is some extra engineering performed on the seat such that one would sink into the seat effectively reducing the effective shoulder width (similar to how crossing arms reduces shoulder width).
3) Perhaps CX added a slight amount of pitch between rows (even an 1/4th inch would be noticable difference)
Obviously, if the seat next to you is empty or sits a loved one or a child, then it obviously won't be nearly as bad as having a bulky stranger sitting next to you.