I don't play fast shooters, but I do play games. The biggest effect on accuracy for me was getting the frame rate high enough. If the game stutters you can't hit anything.
I used the Logitech MX518 for a while. I never got to liking it but didn't know why. Then I bought a 10€ laser mouse with 1000 dpi at the local electronic store. I've been using it ever since because it's much more comfortable for me, probably because it's bigger and the shape just fits better in my hand. I think the better shape leads to more controlled movement. I like the higher dpi because I don't have to drag the mouse too far, but 1000 dpi is enough for me. I would put comfort and ergonomics first, performance and features second.
I also have a Nostromo
. You can use macros, but I was too lazy to program it to anything but WASD. I don't use it anymore either. If you use macros in games now, maybe through LUA scripts, you might give a gaming keyboard a try.
All gaming centric accessories I used were underwhelming.
If ask your question on a gaming site you might get a totally different answer.