Like I had said, even if it puts a damper on some of the file sharing, that is better than nothing. Say currently there are 1 milion ppl sharing illigal files (i knows it more, but nice round figure).
Stats from netsplit.de active: 457 networks, 1269933 users, and 613729 channels on 4460 servers. I sure hope 1 million of them aren't currently filesharing. Thats ~ 78% of irc users unless you mean 1 million people sharing by any means (irc, p2p, etc)
Codemastr: Instead of limiting /fsend and /dcc packetsize why not disable DCC for unregistered copies all together. Or make it impossible to change the dcc ignore options on unregistered copies of mIRC. Then ignore all executables and compressed file formats by default.
To truly make a dent you have to have networks (large and small) make the effort to eliminate it. Unfortunately there are too many admins that get off on their user stats. They don't want to deal with it because doing so will squash their numbers.