I am sorry but no, in practice the message was not always seen by the server. On exit, mIRC v6.35 sends the QUIT message, closes the socket, and exits immediately, in the same way as mIRC v7.41, and no versions of mIRC have ever, on exit, waited for the message to be received by the server.
Also, please note that this has nothing to do with the send queue. The send queue is empty. It it is the underlying operating system and network transport that is the cause of the issue. You can find a technical explanation
here.
As I mentioned, many users disliked the /disconnect delay when it was added in an older version, that is why it was reverted. It is very likely that users will equally dislike trying to exit mIRC and finding that it just hangs around for five seconds. And even this would not guarantee that the server receives the QUIT message - the only way to guarantee it would be to wait as long as it takes to receive a message back from the server, which could be anything from a second to several minutes. So I am not inclined to add support for this. I will however add it to my to-do list and may look into it in future.
