Yes, most X800 PROs are prevented from unlocking the 16-pipes because the die has been laser cut, but some VIVO models of the X800 PRO don't have the laser cuts and can therefore be modded to X800XT specs.
Still, I'm not sure if it's specific to a manufacturer or anything, you should ask around at the Rage3d.com forums, those guys will know for sure.
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<b>Geforce <font color=red>6800 Ultra</b></font color=red>
<b>AthlonXP <font color=red>~3300+</b></font color=red> <i>(Barton 2500+ o/c 416 FSB @ 2288 Mhz)</i>
<b>3dMark05: <font color=red>5,275</b>