I don't like toggling for ADS. I'm much more comfortable just holding the RMB and releasing when done since I'm usually never aiming down sight for more than a second or two (seems more responsive this way for me). I do like toggle for crouch though as I crouch for much longer durations (several seconds).
I'm not a pro gamer so I can't contribute to your second question. Personally, I have all my graphics turned up to max if possible. I love the eye-candy and would rather play the game the way the developers intended it to be played than get a slight edge in a shoot-out with someone else.