I hope I´m not too late, but it just seems to me, Thomas, that you don´t understand what the shutter speed is. It has nothing to do whatsoever with the compression; it doesn't affect your GOP's or any of that. All it is is how long (in fractions of a second) the camera opens THE SHUTTER so light can come in. Just reinforcing what Matt said, it may "look" to you like the resolution is getting better only cuz you're capturing a more frozen image as you go up in your shutter speed. Basically if you shoot a football in mid air with a very low shutter speed, your frames will look have a brown streak going through them, whereas if you shoot the same ball at a high shutter speed each frame will look like the ball is not even moving (commonly referred to as a frozen image). That's all... no resolution change...
Hope it clears things up,
Yecid (all the way from Bolivia) Jr.