Nice, super simple approach. I'd be curious what performance would look on the scale of something like an RTS, but I'd imagine something like an RPG with maps like this would be totally workable. It's just an image after all. :)
Viewing post in How the field of view fog is achieved. comments
Yeah as you mentioned it's just an image, so it should be really cheap to render this even on very big maps. The performance depends more on the FOV algorithm you use and how often you update it. The one I'm using is pretty expensive, O(n^3), where n is the radius of the vision circle. But since the radius is pretty small and I only run it on the player's location, it's not too bad.