Which exploits would you be referring to exactly? The buffer overflow bug which affected pre-6.x versions of mIRC relies on the IRC server being the culprit, unless you're saying your server is trying to use this exploit then by making people change to another server you are, if anything, increasing the chance of that exploit being used. The $asctime overflow bug affecting 6.0-6.02 is IMO largely exaggerated, it relies on the script using $asctime on text from outside sources without any checking whatsoever. I'd have to go with b and I think everyone would have to say c also.
