With road brakes, overfilling have it's advantage to provide a sharper bite, however on some MTB brakes, it's a little too sharp, sometime I have to open the port and squeeze a bit out and close it (while squeezing), this open up the piston a bit more.
Not to my knowledge, too much oil in the reservoir perhaps?
Perhaps a different rotor might be worth trying, not all rotor are trued out of the package.