I've got an X800 Pro and I'm pretty certain that the hardware mod is the only way to unlock the extra 4 pipes. RivaTuner won't do it. In fact, I'm not a big fan of RivaTuner as it has tended to make my cards unstable in the past.
So, if you've got big enough balls and you want to start painting conductive paint on your £280 graphics card, carry on. I wouldn't, I've had too many bad experiences with knackered graphics cards to last me a lifetime!
Good luck!