That's only for some games, IIRC, and emulation as a rule is generally less reliable.
@bonusbarbus: It's very possible that the XBOX360 is optimized for lots of little textures being used, rather than a few slow ones like Freespace 2 is optimized for. For instance, the XBOX360 might have wicked fast pipelines for getting graphics to the processor, but once it gets to the processor, the accessible memory and algorithms might be set up for a very small texture. Think of having a GeForce 8800GTX with 8MB of video memory while running FS2 off of a ram disk.