As others pointed out, with a high quality music source and clean power you can go way beyond the 400W.
I've never measured the wattage, but went to over 100dB with peaks at ~110dB in a 5000 cubic feet room; then people started to leave as it was too loud. No signs of distortions or any other artifacts. I LOVE my M80s!
All I can tell you (although this doesn't mean much) is that I had my volume at 6/10 on my preamp with monoblocks rated at 350W but with high gain - 32dB.
I would much rather keep an eye on the amp than the speakers.
M80 v3, 2 x Emotiva UPA-1 Monoblocks, XPA-5, UMC-1 PrePro
Pro-Ject RPM Genie, Carver C2 Phono Preamp