The 4 speed really didnt like downshifts at WOT (eg, floor the pedal and it would kick down to 2nd) as the 2-4 band would wear out quite quickly (thats why a lot of of the 4spds have a harsh upshift and delay/flaring on downshift) also the 3/4 clutches would wear out often. BUT that being said they were quite tough in terms of abuse and rarely "let go" after burnouts etc, it was more that they gradually wore until the poor shifting was quite noticeable even under normal driving.
on the other hand, the 5 speeds are designed to cut fuel and power under full throttle shifting, eg thats why it sounds a bit like a manual if you are accelerating fast and when it upshifts. that will make the clutches last longer as they dont have that strain under full throttle gear changes.
BUT - the 5l40e box maximum torque rating is 340nm, the 190 kw motor puts out - 340nm.
So go figure, as good a box they are, they are probably designed for something smaller than a commodore, put them behind a v8 and they would fail, unlike the 4l60e's which is virtualy the same box on the 4sp v6's and v8's. not surprised we are starting to see them fail after people abuse their sv6's and calais'.