Thanks everyone for being helpful on this thread. If anyone comes across this discussion, the bottom line is that the modded GBS-8200 board is only capable of downscaling 480i to 15Khz. In the Xbox360 situation, this requires the component outputs. How absurd it may seem, stringing a stock GBS-8100 to scale 480p to 480i into the modded GBS-8200 (via the CGA output/input) works.
I got hold of a UVC board and I simply find it more convenient as all of a sudden I can plug in nearly any source and as an added bonus I can do 24Khz.