You will find there is no optimum split size. That's because rendering complexity doesn't actually depend on split size, it depends on markup complexity. Split size is just an easily understandable crutch the original developers at Adobe used as a way to get *most* epubs to work well with their rendering engine.
calibre does try to split at logical points when those are available. You will discover that for books where the splitting does not happen at logical points, the books have no machine identifiable logical points to split at.
Finally, the extra markup in kepubs is almost certainly there to allow for annotations and bookmarking and likely serves no other purpose. Apparently the developers at Kobo, like the developers at Amazon with KF8 (which uses a very similar scheme) chose not to implement the EPUB 3 CFI spec.