One benefit of an amp is that the headroom comes in two aspects 1) usually more power than an AVR 2) that power is not shared with as many channels as those in the AVR, AND you are not driving those channels from the AVR anymore. There is possibly no real world confirmation on the output of your AVR with all channels driven, and it really doesn't matter because you will almost never have a condition where you are using all channels (unless you run some dumb all channel stereo mode). I would tend to agree with the 75-90 range as realistic. My old Marantz 8300 was rated at 120w and bench tested at 82w ACD, which was pretty good at the time.
What it all comes down to was already mentioned - if you aren't hearing volume limiting or distortion at the loudest levels you listen at, then an amp will not "transform" your sound. IMO, it is a common misconception that an amp makes things sound better. People seem to equate "louder" to "better". The truth is, it sounds exactly the same just louder. If you are actually looking for louder, then maybe an amp is for you though.