Q-pel, GMC, and full quality first-passes

[Katie Boundary]

In an attempt to see if Q-pel, GMC, and VHQ wide-area searches were worth using, I ran a bunch of XviD first-passes with different settings and compared the resulting file sizes. XviD first-passes have a constant quantizer (2), so anything that improved compression should have resulted in smaller file sizes. To my surprise, all of the files had the exact same size down to the byte.

This reminded me of the "full quality first pass" option, and the fact that it resulted in larger file sizes despite using the exact same quantizer. And then I remembered that it also resulted in slower encoding times, and something something might not be fully mpeg-4-compliant.

So my question is: Does an XviD first-pass disable all those doodads, regardless of what your actual settings are? And does "full quality first pass" mean "screw that, use the settings that I tell you to use"?