Good question. The difference between graphics settings does affect the amount of cover that is displayed. On a higher setting the vegetation is thicker and harder to see through. On a lower setting the grass appears less dense and is therefore be easier to see through. However, it is dependant on what your opponent has set his graphical settings to, not what level you have your settings at. On a side note, if the server difficulty you're playing on is set to 'recruit' or the 'enemy tag' option is set to ON, then your oppo is going to see a small coloured blip superimposed over your character anyway, reducing the benefit of whatever cover you're in.
Hope that helps, if you need any more help then feel free to ask!