Well, it definitely works. I played a quick game as France on Earth 18, where I built the GW, then spammed a bunch of spies. I can definitely prove that it affects plot culture, since I plopped a spy in and saw tiles flip to me. Plus, when London and Berlin culturally revolted to me, they both started out with about 1000 culture. Continuing on, my next test with the game is now that they've both flipped to me (London pretty early, Berlin in about 1100 AD), Berlin's culture stretches barely to Germany's next city, so my next goal is to spam the spies there, see what I can get from it.
Unfortunately, on Earth 18, the cities are a bit too close together, so they flipped too early to have them pop up with too much culture in them, but I could definitely see some cases where opponents' capitals are just barely in reach, and you can pump enough EP to really spread culture quite heavily.