WP HTTPS works great in doing what he promises to: converting any link to absolute.
However, sometimes it could be better to have "relative" schema: that is, replacing http:// or https:// with just "//"
This will make links much more stable: across domain-moving and caching (the same page can be cached for both http and https with no issue).
I am concerned about this for caching optimization, indeed: I am setting up a varnish proxy in front of wordpress, and I'd like being able to serve the same cached object both for http requests and https requests.
This seems quite easy to do: just some change in replace_http_url and replace_https_url.
If you are _really_ willing to accept patches about this, I'll consider writing a patch.