The gorilla arm http://www.catb.org/jargon/html/G/gorilla-arm.html will hit you hard if you try to use a near-vertical touchscreen, or hold your hands up and out to make gestures, for long periods. A better solution would be a touchscreen pitched at an angle like that of an eazel or a pitched writing-desk: as those historical precedents suggest, that seems to be a good compromise offering easy manipulation without as much of the hunching over that a flat desk inflicts on you. Or if you just want larger ratio of physical to on-screen movements you could just lower the sensitivity of your mouse.