i have been shooting tethered with just sending small jpegs to the computer, and writing raw to the cf card. its very fast, under a second transfer and viewing to a macbookpro 2.4.
although the jpeg is small, its still twice the size of the 17inch screen, and you can use it to check focus- not pixel sharp- but ive found thatyou can see the sharpness pretty well.
this is the only way ive found that works with the speed of the mk3. and its worked well with dpp/eos capture for over a dozen shoots with me now.
but canon software is very, very dodgey. crashes often, for ex, ive been doing car rig shots and controlling the camera with the computer with eos capture. but if i use live view, it crashes straight after every time. when you fill a cf card and need to replace it, it crashes.