This is an error prone solution, since there's absolutely no guarantee that your connection will consistently be cid 15. Even with /scon numbers (the proper indexed values), it's error prone to assign events to an indexed connection. You should never even be writing "if ($cid == 15)", let alone "on *:TEXT:*:#:15", this is just bad practice.
Instead you should be checking $network (or a combination of $network and $nick if you have multiple connections to a network) for proper support. It would be "neat" if mIRC supported network names there, but given the amount of complexity you'd be adding to the syntax with such a feature, I really don't see the problem of just writing an extremely simple if statement.
