I need some information

Okay so I'm a level 16 roguehttp://oi50.tinypic.com/2vdqk3q.jpg my skill build is Shadow Peircer 5/5, Aimed Shot 5/5, Razor shield upgrades 1 & 3, and SSS upgrade 3.

Question 1: Why do people use upgrade 3 on shadow peircer and nox? They do very low damage so why?

Q2: how do rogues hit high crits? I was in pvp and someone said they crit for something in 600's or 800's and then I find out this is "low". My pride isn't damaged at all knowing I've never crit over 500... does the bonus damage tell how much higher your crit will be over your normal attack? Or what?

I thought I had more than that but that is all I need for now.

Answer 1: People have personal refrences resulting in their own crazy build.
Answer 2: Aimed Shot stacks up your critical hit percentage by 10% I think resulting in a very high crit ( about 40 - 50% )
Crit is the chances of 2x+ your normal damage.
For example: Your damage is 100 but you have a 15% crit, if the critical just so happens to occur, then your damage will rise from 100 to somewhere are 200 or so.( most likely a bit more )
If someone had 50% crit chance ( probably with pro gear, pet, and Aimed Shot ) his chances of criting will be very high meaning he will have more of a damage output when the crit occurs ( a 50% chance )
Hope this helped bro! :)
-Skull out.