My static and I attempted to start killing these as soon as level 51...that was a really bad idea. They are pretty evasive, as exp mobs go, and our melee couldn't hit them reliably. What's more, they were ripping up our PLD with Sickle Slash. We stuck with them, somehow, and now we're 53-54, which seems about right for these. I'm guessing exp will be decent until 55-56, but we'll likely move out to Aht Urghan before then.
As for the coffer key drop-rate, we didn't get any for the first two hours of our party ( this is with TH2, we have a THF in our static ) but we got three in a row towards the end.
My only advice would be this: try and keep your tank's HP above yellow. Anything lower than white and they could be killed by a lucky Sickle Slash.