My goal is to maintain a web file server separately from my main ASP.NET application server for better scalability. The web file server will store a lot of files downloaded by users.
So the question is: Is it worth to adopt FreeBSD + Apache + ZFS, or will good old IIS be сonvenient enough?
It all depends on your skill level and how much load you are getting on your servers.
If you have spare (physical) resources and have the technical skills and experience to maintain production machines running different operating systems, I'd recommend going running lighttpd on either Linux or FreeBSD. A light OS install with a static file optimized server will perform faster than Apache or IIS on a heavy OS.
However, unless you are extremely comfortable with these solutions, just stick to IIS on Windows. Move the static files to their own machine if you have sufficient load. If you aren't currently thinking about multiple ASP.Net frontends, there's probably no need to spin off the static files yet unless we're talking multiple gigabytes of files.