DITTO!!! The M6 is LOUD with TONS of headroom!!! Honestly I couldn't imagine anyone needing more.
I don't have any experience with the SD800, however I was trying to decide between the M9 & the M6 myself. The reasons to get the M9 over the M6 wouldn't necessarily be for the extra headroom. As you probably know, the M9 has an excellent onboard compressor & a 9-band graphic EQ. The graphic EQ on the M9 has an on/off switch, and I noticed that even with all the sliders set flat on the graphic, as soon as you switch the graphic to the "on" position, the tone changes and sounds a bit compressed and more distant compared to having the graphic EQ switched off. The frequencies provided on the graphic EQ are great & it works well for tone sculpting but honestly, there are so many great tones that can be dialed in on the M6 without the graphic EQ that you don't need it. The change in the inherent tone of the M9 when switching the graphic EQ on was a deal breaker for me.
Since the M6 is plenty loud, and I wouldn't use the graphic (because of the change in tone) or compressor, I opted for the M6 which I picked up last Friday, and couldn't be happier with my decision.
Like Neil said, I can't imagine anyone needing more headroom than the M6 provides - in heaps!
The M6 is a tone monster and just plain rocks!
-Rey

