So, we reached the new local users record for insp - ~12500 bots, and it wasn't struggling to run at all.
This was running on 1.1.17+SVN code, backported from some of my work on 1.2, which contains vastly improved code for quitting users (no lag, instead of lots of it).
The results speak for itself, really ;p
In other news, fez has replaced IOCP on windows with select (hopefully temporarily), which will be used for the 1.1.17 release. It seems to fix a number of problems that windows users were encountering, which is good news.
If you are a windows user and have had problems in the past (or indeed, still are), we encourage you to give http://www.inspircd.org/downloads/InspIRCd-Devel-1.1.17-Select.exe a go, and give us feedback.
I'd like to thank RockerMONO for his extensive prodding and feedback on this, and most especially fez for his help in getting it working.
Hopefully we'll be able to fix IOCP at some point.