It seems that I'm getting close to the performance requirements I set to the core.
Transfer times for 800mb file, when daemon is working as both client and server:
1 connection: 6.3 seconds (253mb/sec)
2 connections: 12.6 seconds
10 connections: 62.6 seconds
100 connections: 619.8 seconds
1000 connections: 6285.2 seconds
Performance of single cached transfer seems to be always constant.
Just one odd thing.. I noticed I had pulled the wrong figure for old core performance:
http://www.ioftpd.com/board/showthread.phpthreadid=3174
... and the odd thing is, that now that I try to transfer same file, I get lower performance (even with the old io) And I can't remember any changes since (other than I added 1Gb of memory)