I'm using ecryptfs for an encrypted home directory. I would like to try out the mount option "ecryptfs_xattr" on my encrypted home directory, because it will probably improve performance. Can I specify this option somewhere, and still have it decrypt the home when I log in? (I assume I have to re-create the encrypted home directory, that's no problem)
First of all, it's extremely doubtful that you'll see a noticeable performance improvement using xattrs for eCryptfs metadata.
As for specifying particular mount options, you can sort of do this using the "ALIAS" feature, which I've documented in the mount.ecryptfs_private manpage. Here, you can add some fstab-style mount options, which can work for other eCryptfs encrypted directories, but unfortunately not $HOME. The reason for this is that if you mangle these options, you could render your $HOME directory unmountable, so we've restricted the options you can tweak for encrypted $HOME. Sorry.
Full disclosure: I'm one of the authors and maintainers of eCryptfs.