Thank you for the great plugin. It is working wonderfully on our site.
Have you considered a Google New sitemap module for it to extend it's existing functionality.
Essentially it is a sitemap that includes the posts/pages from the last 2 days, < 1,000 per sitemap.
You can read more about it here:
While there are a number of existing WordPress plugins that generate them, it would be nice to have a centralized solution that takes care of all of our sitemap needs (which your plugin does well) as well as awesome caching functionality!