iApp for Exchange CAS Load balance Method Doesn't Works for OWA Pool

Which version of BIG-IP are you running? Are you using TMG for both forward and reverse proxy, or just reverse?

LTM, APM, and AFM can do a reasonable job of replacing TMG nowadays, especially in v11.5. In the long term, I recommend going that way. For now, you should be able to work around the OWA persistence problem by inserting the X-Forwarded-For header in TMG (you'll need to disable this feature in the HTTP profile on BIG-IP), then changing the OWA and default persistence methods in the Exchange iRule to use that header value:

    ...
    "/owa*" {
     Outlook Web Access
    if { [HTTP::header exists "APM_session"] } {
        persist uie [HTTP::header "APM_session"] 7200
    } else {
        persist uie [HTTP::header "X-Forwarded-For"] 7200
    }
    if { [HTTP::header exists "Accept-Encoding"] } {
         HTTP::header remove "Accept-Encoding"      
     }
    pool exchange_2010_owa_pool7
    return
}
...
    default {
     This final section takes all traffic that has not otherwise
     been accounted for and sends it to the pool for Outlook Web App
    if { [HTTP::header exists "APM_session"] } {
        persist uie [HTTP::header "APM_session"] 7200
    } else {
        persist uie [HTTP::header "X-Forwarded-For"] 7200
    }
    pool exchange_2010_owa_pool7
}
...