I doubt (but have no evidence to disprove) that simulated child porn causes viewers to act on those desires in the real world.

It's not as if people will be "converted" to child porn by viewing these images; rather, it will be viewed by people who already have a predisposition to such content. My gut says it might reduce real-world incidence if they can get their rocks off to digital content, rather than feeling compelled to do so in reality.

But as you pointed out, there is very little credible research on this topic, so it's hard to say conclusively.