The default crosshairs are very big and misleading. The lmg spread is smaller than the LMG crosshair implies so you might think you're hitting when really you're miles off. The same applies to both the pistol (no spread, but the whole circular crap can be misleading), hmg and the shotgun. I use custom crosshairs for all my weapons and when I changed I saw a significant improvement in my aim.
The crosshairs are for more accurate and you know if it's on the alien and you're shooting then you're hitting. Furthermore i find they obsecure my screen less. I remember when i played with the default crosshairs again and i found i couldn't see things very well. I did have a pack of crosshairs uploaded but my site is down unfortunately.
High sensitivities are just in general a bad thing as they make it harder to track. Even slight movements can lead to your curser being no where near the alien and makes it harder to control. Back when i first started on LM i played with a sensitivity of about 16 but now i'm on 2.8. Skulks, particularly at distance do not appear to move as much on your screen and a lower sensitivity with more controlled actions you'll find tracking far easier.
Just in general you'll find tracking easier if you drop your sensitivity. I've been slowly dropping it till i found something i was comfortable with and i've settled in 2.8. I find i'm able to move my mouse at a speed fast enough to track anything on NS with controlled movements. Any higher and i found i was having to compensate for moving the mouse too far and was often trailing behind or infront of the skulk. If this was the case i'd tweak i sens accordingly.
I'm not saying you should drop that low, this was the result of 4 months of competitive NS, tweaking the sens god knows how many times and a lot of evaluating of my own play. Whilst my drop in sens helped improve my aim i also improved my reflexes within the game and my NS brain. For experimental purposes though i'd recommend cutting your sens in half or to a number like 7 and seeing how it goes.
You have to stay open minded and be willing to play with it for a while but i'm confident you'll understand the advantages of it after you give it a go