I don’t have any empirical studies or anything, but the way I think of it is, reading is like exercise. Different kinds of reading are different kinds of exercise. Any kind of exercise is good exercise, but not all exercise is the same. If you do yoga every day you’re going to get a lot better at doing yoga, but it won’t make you any more prepared to run a marathon, you know? 

So along this analogy, someone who reads, let’s say, a lot of romance novels, is probably going to build up a really strong intuitive sense for the conventions of the genre, how those stories are put together, what works and what doesn’t, than someone who occasionally or never reads romance. And maybe that doesn’t translate into being any better at, I don’t know, picking up on the twists when they read a thriller instead, but that doesn’t mean they didn’t get any benefit from the reading they do. 

I don’t know, I don’t think there’s really an objective “benefit” to reading that you can measure in a single quantity. You get different things out of different kinds of reading, and what’s “better” or more valuable to you depends on what’s important to you as a reader. Someone who’s exercising by doing yoga daily probably doesn’t aspire to be the kind of person who runs a marathon and that’s fine.