Tyler Gaw documents his process of moving off Sass and onto PostCSS, but keeping most of the code the same. That meant making sure he was using PostCSS plugins that would replicate most of Sass' functionality, like nesting and mixins and whatnot.
Tyler is sold. I find it an interesting experiment, and it's cool to know it's basically possible, but I'm definitely not sold yet.
Now instead of just having Sass as a dependency, which is an active healthy project, you have a whole bunch of plugins with different authors as dependencies. And for what? Assuming you use libsass, you don't get any speed. If you like some particular PostCSS plugin, using Sass doesn't prevent you from using that also. One thing I definitely wouldn't recommend is preprocessing those custom properties, as those are not interchangeable things.