I don't know where you're getting your numbers from but the M400 delivers power significantly in excess of that. From the Hi Fi News review:
In practice it bests this by some margin at 430W/8ohm and 810W/4ohm with a dynamic output of 475W, 940W and 1775W into 8, 4 and 2ohm and with a scary 3010W (that’s 55A) into 1ohm
So there's no question about the M400's ability to deliver monster current in excess of the listed specs.
And as you said, if we've not seen independent Gryphon measurements we really have no idea how it stacks up in the real world.
I trust measurements, not specs. I bought the M400s knowing exactly what I was buying.