I don't hear any noticeable difference either. After every update I hear people go best ever! My presets now come alive! Like a blanket has been lifted off my speakers! Me, it just sounds the same. I don't even bother to adjust my old presets. I think I haven't changed them since what? Q.7? Then again, I must have built dozens of overdrive and big muff pedals before I got into the Axe FX, and to me most of them sounded the same as well. I would try out a new pedal build and go 'meh, sounds exactly like the one I was already using.' In the end I probably did it more for the fun of building things then for an actual quest for tone.
So color me indifferent whenever there is a new firmware upgrade, as I expect it to sound exactly the same. Color me even more indifferent when a new amp model gets added, because to me most amps are like most overdrive pedals, all the same as well. Color me very excited though when a new firmware upgrade comes with a new effect. But that only rarely happens.