I have had few clients move 100% to HTTPS and nothing bad happen as in going backwards and I won't lie I haven't seen great gain from it either. I haven't heard of anyone getting great gains from it but not moving the whole site to it is most likely doing more bad than good. Also if you have gone backwards after moving to HTTPS there could be errors. too.
I would do the following:
- Move the whole site to HTTPS
- Check that HTTPS is setup right. Can do this at http://www.webconfs.com/http-header-check.php
- Review any errors in webmaster tools or use a tool like semrush to check for errors.
- Insure all 301 from http to https is setup right and leave it for a month and see what happens.
