from my experience..i would ditch the 3.90 R&P mod in the r160. i blew 2 r160 diffs before i upgraded to a a full r180 rear end. this was in my old wrx.
the whole point of upgrading to a 6mt is to have fun and not really worry about breaking any drivetrain parts besides an axle..lol. the r160 can not handle hard shifts and/or donuts/drifts.