It's technically supposed to be 1.5ms center, but 'center' is arbitrary anyways, you'd have to measure it using a protractor and a long arm. I've seen some throttle servo's where the control loop was tweaked so that is was very non-linear, it was on a really old car before even halfway advanced electronic mixing methods were used. If you want a little extra torque out of your servo increase the frequency as high as you can until the servo starts acting strange. The higher the update frequency the greater the immediatly available torque the servo gets because it's getting it's power pulses more frequently. So called digital servo's do this in a slightly more advanced way. It's a good way of partially bridging the performance gap between analog and digital servo's without dropping the cash on digitals.