I would not chooose the XBR for 1/12 for two reasons. May need a receiver pack for 4-cells, and istaller than the GTB 4-cell version. This means if you are racing the GT style bodies getting the ESC to fit will reqiure some body work or raised body.
For 1:12 I would either use a GTB 4-Cell or the LRP Sphere comp.
For motor choice, go with the 13.5 SS PRO, the XBR will need to be upgraded to be competitive and that just cost more money than doing it right the first time.