2xs,
A nice, detailed report! It sounds like your Onkyo is unable to drive the M80 properly. Also, it does seem that your M80's are normal. The SPL level of 100dB is fairly loud, but yes, it should go much louder without a problem. The "peak" SPL (if you have the digital version of the RadioShack meter) should readily go 110dB if you pump it out.
One additional question: Did the receiver shut itself down as soon as you turn the volume knob up to 83 or 90, within a few seconds? Or did it take 30-60 seconds or more before shutdown? If the former is the case, it definitively suggests the over-current shutdown (the latter suggests thermal).
At any rate, before jumping on to a power amp upgrade, I would still request Onkyo to "repair" the receiver under warranty. I do not think your receiver is functioning as it meant to. I believe that the protection circuit in your particular unit happens to be set too conservatively. I say this because I understand that the early batches of the current Pioneer Elite receivers had lots of shutdown problems just like yours, and in a majority of cases, Pioneer could "fix" it upon request, without ever admitting that there was a problem in the receiver.
