From my experience it's depending on overall performance. A high-end player on a 515 alt will perform great. An average player in 515 gear will have to work at it. I would wager that for the majority of guilds out there, Flex is another form of progression, not simply something to do with alts on off days. After all, if you can't down a boss in Flex then you have no business trying that boss on Normal, so Flex becomes a training simulation for the fight on normal mode.
Flex might be easier than normal, but I wouldn't exactly call it "easy" by any means, and in fact thinking Flex is some joke mode where you don't have to pay attention is a sure-fire way to wipe.