Hmm heres my take on this.
Last month my good 'ol trusty MX518 went bad on me. It just stopped working out of the blue. Till now im using a cheap USB mouse and the following is my inference.
The MX518 is almost twice as accurate as my normal USB mouse, atleast it was in the beginning. After a month of extended usage my K/D ratio is back up where it was with the MX518. But this does not mean my 200 wala USB mouse is better than a 1.6k G400 or 1.9k Deathadder . I do miss changing my DPI on the fly [for CQW and LR combat] and the extra coupla buttons which helps fragging and secondary firearm switching that much easier.
So the idea is, get a gaming mouse within 2k [Mx518, G400 (1000hz polling-better], Deathadder] it really helps u hone ur accuracy. But just like anything the mouse is a tool, its ur skill that really matters.
P.S 1000 DPI is more than enough given that the pros really play with 200-800 DPI. But it all comes down to your play / grip style. If u are the frantic, borderline-meth-head-twitcher with a claw then u need a minimum of 1800 dpi, if u play methodically with calculated moves with a palm then anything below 1800 will do just fine.