Most efficient way to write this

The most efficient option might be to get a new IP address to resolve all the alternate spellings to and then use however complicated an iRule that you want.

Else, if you need to stick with one address, you can put the real hostname first in a switch statement and the rest afterward. That way you would only do one evaluation of the host header value for the majority of requests. I don't think evaluating the host header several times in a switch statement would add that much overhead anyhow, but it's always good to try to make things as efficient as possible.

 
 when HTTP_REQUEST { 
  
     Check the requested host header (set to lowercase) 
    switch [string tolower [HTTP::host]] { 
       "realdomain.example.com" { 
           do nothing 
       } 
       "alternatespelling1.example.com" { 
           check the URI and/or redirect request 
       } 
       "alternatespelling2.example.com" { 
           check the URI and/or redirect request 
       } 
       default { 
           no match, take some default action? 
       } 
    } 
 } 
 

Aaron