It really irks me to see the constant barrage of DPI nonsense that manufacturers put out. I hope this can be useful information to help understand why mice with ultra-high DPI are hokum.
1. DPI is sensitivity and nothing more. DPI in plain english means, "how many pixels the cursor moves on screen when moving the mouse one inch." Mouse manufacturers like to tout high DPI, but it's a marketing ploy that has very little actual value. If your monitor has a resolution of 1920x1080, a 1600 DPI mouse would require you to move the mouse 1.2 inches to span the entire width of the display. A 4000 DPI mouse would require only 0.48 inches of movement to do the same. Any sensitivity lower than the highest possible lowers the DPI that is being used. It's incredibly unlikely that anyone playing an FPS is utilizing 4000+ DPI.
It's a little tougher to calculate effective DPI in a game like TF2--you'll see different values depending upon your resolution because you're projecting a 3D game onto a 2D monitor and pixels closer to the crosshair will have sharper angles than pixels further from the crosshair.
You can calculate effective DPI in a 3D game pretty closely with the following:
r = resolution
s = sensitivity in inches per 360
f = field of view (usually 90)
(pi * r) / (s * tan(f / 2))
Given the 10 inch example, the rounded effective DPI is 603 if your resolution is 1920x1080. If your resolution is 1024x768, the rounded effective DPI is 322.
There can obviously be other factors in mouse feel, such as positive or negative acceleration, jitter, hand comfort, etc. but in the above examples, any mouse with 603 DPI or higher would be just as "accurate" as the next.
2. Raw input does not disable mouse acceleration. It simply bypasses the windows mouse driver. If your mouse sensor has either positive or negative acceleration, that won't change with raw input.
3. Too many different factors at play to really answer this one. Scout and Sniper have very different aiming mechanics--zoom vs. no zoom, headshots vs general aim, movement mechanics, tracking vs flicking, etc.