It depends a lot on the AI as to what they do with their Holy City. In numerous games I have sent Great Prophets to the opponents Holy cities and converted them, at the same time converting all the surrounding cities with a mass of missionaries. Often this results in me being the only civ with an active religion. Even after dozens of turns, and the pressure in the other Holy cities becoming enough to make missionaries of their own, they don't. The Holy city just needs another 'beat-down' from my GP and everything is under my control again. On the other hand, I have done the same, in other games, to the Holy cities only to see miraculously 2 or 3 Great Prophets appear from supposedly inactive cities (religiously speaking) and start to take back their former cities. It is like it doesn't matter what is the dominant religion in the AI cities, whichever religion that AI started, is the religion of their GPs, regardless of whether there is an active city. So, I can't see a missionary, enhanced or not, making too much difference to the spread of opposition religions.