I've gotten a little over 6000 3d05marks with my pro. Clocked at a rather extreme 580/580 (I only have 2.0ns ram too )
X800XL 16pipes 400mhz =Fill rate of 6400 w/e
X800Pro 12pipes @ 533.34mhz = fill rate of 6400 plus your vertex and pixel shading will be ripping along faster than an X800XL @ 400 mhz will.
Now to shoot my self in the foot.
X800XL @ 430mhz 16pipes = 6880 fill rate,
X800Pro wil have to do 573.34 mhz to keep up with that, but then again the vertex and pixel shaders will shredding anything you feed them. My x800pro (pci-e) With an ATiSilencer can do 580/580 without the display corrupting. I havn't tried any higher than that becuase at that speed I broke 6050 3d05marks.
My $0.02 of data.