[Exim] Re: global killfile

OK, so now I have a system_filter with a block like this:

...
if $header_subject: contains "THISisSPxM" or
        $header_subject: contains "snxwhite and the sxven dwxrfs" or
        $header_subject: contains "cheap vixgra" or
        $header_subject: contains "pxnxs enlargement" or
        $header_subject: contains "Nxked Gxrls" or
        $header_subject: contains "BxCKGROUND CxxCKS"
then
        logwrite "$tod_log $message_id $h_to: FAIL on subject
\"$header_subject:\""
        seen finish
endif
...

Is there a limit to the number of "or"s that can be in here? How much
of a performance hit is it? I can see this growing to be quite large.
Is there a better way of doing this?

Thanks
Skye