It's not about being serious... Lmao sometimes I wonder.
400, 800 and 1600 DPI are all fiiiiine. It's just whether you actually need the extra precision or not. You can tell that they're all distinct in precision though.
I mean seriously, whether I use 400 DPI and 2.4 sensitivity in CS or 800 DPI and 1.2, 800 DPI and 2.0 sensitivity or 1600 DPI and 1.0 sensitivity.. It won't matter much because the amount of degrees you're allowed to move your crosshair still is small enough to do precise long range shots. I use 400 DPI and 3.1 sensitivity in both Quake and CS. If I aim at someone's head from a far distance it won't make me skip over his head or something, so it's fine.
But if 1600 DPI makes you secure about it then that's cool also. I just don't like the extra fine movement from it.
So I really wouldn't bother using a higher DPI unless you prefer the smoothness from the extra counts or you use something really high like 4.5 @ 400 DPI in CS (but then I would question if it should really be set this high in the first place for a game like this).
But nonetheless, I'm really excited that there's gonna be an MX518 with a good sensor and hopefully even better build quality. Knowing Nixeus it'll even be affordable as well.