Can Ifrit(Hard) be done with gigantic lag? Yes, if you step up your play and stop goofing the fuck up. I know I did the ONE time we managed to beat it after a month of harsh fights (and then we never fought him again, hell with him).

However, that relied on a handful of facts: Our DDs were pretty much safe from plumes regardless because of safe spot and melee zone. We all could survive one or two eruptions that we'd invariably eat due to lag. The fight is sufficiently forgiving of a very minor goofup that will invariably occur with lag. Hell, not even reaction lag just the difference of positioning between server and screen.

Now look at the harder battle: Are the safe spots as easy to stay on? Can the ranged DDs just stay on that one spot the entire fight and be 100% safe from plumes? Is there still a melee zone? Is reaction time for Twin Triple Eruption same as before?
If anything, from that image and the hate reset, plumes can be a massive problem, rather than eruption. Do you need to always, very rapidly, run to the nearest clearing in the plumes? If that's true, I can attest i'm going to eat each and every single plume, regardless of my reaction time. Is that going to make it hard/impossible? Who knows.