That's an ok deal I think, but unless you're bent on getting it new from a reseller, the most I'd pay is $400. At EE they routinely show up between $300 and $450, since they're big ticket items as far as flashlights go, they don't move very fast. Just in the last week I saw at least three below $400, I snagged mine (M910 with an additional ARMS dual lever mount) for $350 shipped, without the additional ARMS mount it would have been $315 shipped, now THAT's a good deal
Surefire would warranty it anyway, buying it new only costs you more.
The Turbo Head option, I'm sure it is the big head you're talking about. The M series heads all fit the M9x0, I've used the Turbo Head and an LED with mine. In the picture I posted above, the LED is one on the left. I have two, one on my M6, the other on an M3. Playing with it out to 200yds or so, the beam is tighter due to the larger surface area of the reflector. It is bulky though, and looks funny