[Exim] Re: global killfile

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

...
if $header_subject: contains "THISisSPAM" or
        $header_subject: contains "snowhite and the seven dwarfs" or
        $header_subject: contains "cheap viagra" or
        $header_subject: contains "penis enlargement" or
        $header_subject: contains "Naked Girls" or
        $header_subject: contains "BACKGROUND CHECKS"
then
        logwrite "$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