This game will already be irritating by around 150 ms ping and beyond frustrating by 250 ms ping. I would not recommend it if you can't keep below 300 ms. It doesn't have the best of ping protection, and has only one data center for the whole of the Americas, as compared to the usual 3-5 for NA seen in other large MMOs.

Phone plans seem like your best chance here. Barring that, though, I'd probably end up sticking with WoW as a main and picking side-MMOs based on the ping to their nearest datacenter and their ping-friendliness. No matter how ping-friendly XIV were, though, it's a bit unrealistic to expect any real-time game to be able to compensate well for a full second's latency, especially if you plan to heal or tank.