I'm sure this has been done before but I've never seen it in any config I've used/downloaded. I thought it up yesterday and I've been testing it with pleasing results :) (I don't think this would be much good for low seners, I use around 3.9 at 450Dpi and m_pitch/yaw at 0.018. Also, if you use zoom this wouldn't work either without adding extra cmd's to your binds).

bind mouse1 "+attack; +zoom"
cg_fov "90"
cg_zoomfov "90"
weapon 1/3/4/5/7 "cg_zoomsensitivity 1"
weapon 2/6/8 "cg_zoomsensitivty 0.9"

I'm sure you can work out from this little script that it would give you a lowered percentage of your original sensitivity for the MG, PG and LG 'only' when you are firing. I tried it at 0.75 to start with and found it to be too much of a difference from my standard sens. After a game or two of getting used to it I just hit a healthy 47% LG in a CA game which is on the higher side of things for me. I usualy hit about 34-42%. More importantly than the percentage hit, I felt more confident in LG fights and found a delt more damage than usual. Plasma also felt more effective :)
Try it and let me know what you think?