That said, I believe the enrage timer isn't really the issue with this fight mainly because I don't see people complaining about "getting him to 5% every time." Most threads about LR-5 involve people wiping far earlier than the enrage.
As a fully-geared tank, sometimes I will PUG HM LI just for (masochistic) fun. About 50% of random groups that I get through groupfinder have severe problems with lack of DPS - LR-5's enrage will kick in with 20-30% of the bosses' health remaining, despite the fact that my HP has never dipped below 70% and none of the group members have gone down. After two or three runs like that, I will announce "I don't think this is going to happen with this group" and then leave.