You forgot the part about CAMS being in deep financial sh*t, and needing a return to a cheaper series. And the part about the Sierra's domination which contributed to the Group A formula's infamacy in this country. The GTR's were never "banned", they were just not included in the new ATCC rules, for obvious reasons; they were too quick, and bloody expensive. Too expensive to run save for rich or factory based teams.
Back On topic: If all jap cars were banned? Thats easy. A Sierra Cosworth, or an E30 M3.