Full disclosure, I have never used the rss features of lemmy or piefed, so my answer is not coming from a place of personal experience.

To answer things in turn:

• RSS feed for notifications

This does not exist. I didn't realize this was a feature of lemmy. Is there some kind of authentication you have to do to access this, or is it simply just a big long string of characters so that it is hard to guess? There is potentially sensitive information in an inbox such as private messages, so this is something I think should be handled carefully.

• RSS feed for a user

This does exist! If you go a your, or another user's profile page. On the overview tab, to the right, there is an RSS feed button that has the xml you need.

• RSS feed for saved posts/comments

This does not exist. Looking through rikudou's project, it looks to be lemmy-specific through using lemmy's api. I am willing to bet that, if this is something rimu wants to include in piefed, he would just make it a feature that is built-in rather than adapting an external service like this.

• Vote/comment counts in RSS feed

This is probably possible, but I have never touched the rss code for piefed, so I am not sure how big of a lift this might be.