I'm a little late to MW3, so I'm sure everyone has their XIM3 already fine tuned by now, but just in case anyone is having issues finding the sweet spot for their XIM3 for MW3, I'm posting my settings. Everyone plays differently, so who knows if it will help, but it certainly can't hurt!
I use a Logitech G500 at 5700 DPI and 1000 Hz polling rate. Here are my MW3 settings:
Translator: Smart Advanced
Game: CoD:MW3.1
Sensitivity: 29.00
Acceleration: 0.75
Y-X Ratio: 1.00
Smoothness: 0
Boost: 0
(I use the same settings for Secondary Mouse as well).
The trick for me personally was adding 0.75 acceleration. Once I did that, it felt 1:1 for me. This thing is awesome.
Here's a short video from my first game after finding these settings that I uploaded using the MW3 Theater:
http://www.youtube.com/watch?v=Q55odYQAETw