If I had to guess I'd say maybe iLVL 75 weapons for ifrit extreme and 80 or 85 for Titan/Garuda. I highly doubt they're gonna add other unique item drops for them, but that would be a pleasant surprise. I also wouldn't expect the items to be better than relic +1's yet, even though the fights themselves are going to be a lot harder than any individual relic fight.

They're guaranteed to drop a fair amount of tomes though, so maybe for the extremely geared/talented this would become the fastest way to farm philo/myth.