<p>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.<br>
So the question is: Is it worth to adopt FreeBSD + Apache + ZFS, or will good old IIS be сonvenient enough?</p>

I understand you will serve only static files. In this case, lightweight HTTP servers will give you a higher performance for a given machine. The following are well known:
Lighttpd
Thttpd
Nginx
Many more are listed on
Wikipedia
. There's a more recent
article
on IBM DeveloperWorks.