Just getting back to some basics for comparison;
With a fresh TC install, no Xorg or ati video drivers, but rdesktop loaded: function of redesktop is pretty fast and scrolling in Excel not bad (still a little bit of latency for large mouse scrolls, but short scrolls instantaneous).
Of course this is with having the two monitors showing the same mirrored screen.
So at this stage I would have to presume the difference between the basic setup and my Xorg Radeon 7000/VE dual screen setup must be something to do with either the video driver or something in Xorg?
Is there anything else like Xfree86 that I could try instead?