Quote: "ALSO, let's say I don't want to re-sight my gun, but I want to shoot 180 grain bullets. What muzzle velocity would I need to have a similar trajectory to the 165 grain at 2700 fps? "
It isn't just the trajectory that you have to worry about; it is mainly the point of impact. As stated above, just changing the brand/style/weight of bullet can change the POI significantly and require resighting. Test the different loads and see how close the POI and trajectory matches for your rifle.
Good shooting and be safe.