Long story short: the Yaris sold in the US is a rebadged Mazda 2, so homologating the GR Yaris to sell here would be starting completely from scratch. Doing a GR Corolla at least saves crash testing etc.
C&D guess the GR Corolla will get the usual hot hatch bits like upgraded suspension, brakes, wheels & tires. They think the GR Yaris awd system will be left out in favor of fwd, but a manual transmission will be available. No mention of an LSD one way or the other.