A few personal opinions on voice actresses being added. Over the years I've mostly watched erotic videos with the sound off, for discretionary purposes. But lately, whenever I can, I'll turn up the sound, and I've realized that sound can add a whole other dimension to the scene - IF the actress is good at it, or at least makes the kinds of noises that I like to hear, and matches the visuals. Some porn stars are horrible at it, and really irritating, in which case I just turn the volume off again. But sometimes the right panting and whimpering, leading to high flutters, unladylike groans, bodies slapping together, and other sex sounds, can be sexy all on its own.
So, leaving any cost or production time issues to the developer - why not try it? If it doesn't sound right, I'll just turn the volume back down.