I suspect that you might need to do the 3 wire trick to get your Xbox to boot. The problem with the Gentoox flasher is that it only flashes the first 256kB - the option to fill the TSOP with multiple copies of the BIOS hasn't been enabled (essentially it seems to use Raincoat 0.5 in its default mode instead of Raincoat 0.7, where auutofill is enabled by default).
Here is a picture of the 3-wire trick wiring points, but I can't find a post about it, so you'll have to do a bit of searching. The short version is that you ground A18 and A19 to make the Xbox boot, then remove the ground from A18 and 19 and reflash the TSOP with a 1MB BIOS, or using a tool that has the auto-fill option enabled.