If that's the case, then the enrage timers are probably a bit too tight. Enrage timers, especially in small group dungeons are a pure dps/gear check. They rarely promote good mechanical execution....and are the primary reason players prefer to "overgear" the instance.
LR-5 has a soft enrage at 3 minutes.
He has 389,304 health at start.
This works out to 2162 dps between the tank, melee and ranged dps.
As long as the team brings someone with an armor reduction debuff (and there are several classes that have it, including melee), that should be doable with a tank and 2 dps with Tionese gear if they execute the fight perfectly.
The only old MOX parse I have of a boss fight back when my commando dps was in Tionese was on False Emperor. I hit 750 on the Malgus fight, which has a lot of downtime. And this was with an average rotation without the 4-piece set bonus (free HiB). A properly executed LI fight uses my HiB and DR when moving, so it doesn't have near the downtime of the Malgus fight. I could have hit 850 on an LR-5 fight in full Tionese on a well-executed fight.
If the other dps hit 850 as well, that leaves 462 dps for the tank -- easily doable in Tionese.
And the enrage isn't a one-shot either. The plasma balls drop faster at full size and the grates all have lava. If LR-5 is around 5-10% when he enrages, it is still possible to dps him down.
However, at that level of gear, all it takes is one mistake:
Miss an incinerate -> wipe
Miss a plasma arc -> overtax healer probably a wipe
Drop an electric coil on the tank -> wipe
Fail to gather and aoe adds -> wipe
The soft enrage timer can still be beat as long as you have the mechanics down.
But as I've said before, give 4 unskilled players a full set of Best In Slot Campaign/Black Hole gear and tell them to run LR-5. They will wipe.
It's a mechanics check. It takes skill.
I would also say that most people that have issues with LR-5 wipe before the enrage timer. Heck, I died 14 times on LR-5 before I beat him, at anywhere from 30% to 70% health. It wasn't until I ran the fight with someone who taught me what to do that I beat him.