While I don't have a really solid solution for you, I do know that GSync can work in borderless windowed mode; I'm running in that configuration.

THAT SAID, Razer's "game boost" thing has sometimes erroneously gotten confused by stuff and caused my game to drop to like 20-some FPS for chunks of time specifically in borderless windowed. If you happen to have a Razer keyboard or gaming mouse or whatever, I might temporarily quit Razer Catalyst (or disable the "Game Boost" mode before starting FFXIV) and see if that improves it.