Can this redirect rule be optimized?

Using a switch is one of the most optimal ways to do this. But if, as you said, you foresee manageability issues with the large number of domains across multiple virtuals, then you could do it with data groups. Here's an alternate implementation using a string datagroup with the domain mapping contained in the data group.

class domain_mappings {
  "domain.ca domain.myasp.com"
  "domain.com domain1.myasp.com"
  "domain.biz domain2.myasp.com"
  "joe.com domain3.joe.com"
  "fred.us domain4.fred.com"
}
when HTTP_REQUEST {
   Extract the domain portion of the host
  set valid_domain [regexp {\.([\w]+)\.([\w]+)$} [HTTP::host] domain company ext]
  if { $valid_domain }
  {
     Look for match in the domain_mappings data group
    set new_domain [findclass "$company.$ext" $::domain_mappings " "]
    if { "" ne $new_domain } {
       Issue redirect
      HTTP::redirect http://$new_domain[HTTP::uri]
    }
  }
}

There's probably some more optimal way to extract only the last two tokens from the host aside from a regexp but this should account for any type of host you specify (www.foo.com, www1.www.foo.com, etc).

So, with this option, you would either maintain a single master domain_mappings list and apply the same iRule across all of your virtuals, or split out the domain_mappings data group into smaller more manageable ones. Your choice.

*disclaimer: This is probably lower performance than your switch option, but it is a much simpler version for when you scale to a larger format.

-Joe