Okay, here's what I get. I couldn't try it on Efnet for some reason, maybe the servers I was able to get on had part/quit reaosns turned off. I tried this on my own test bed and got:

Part (mIRC default):

[21:53:37] * Parts: Plod ([234567890[234567890[234567890[234567890[234567890[234567890[2345...)

Part (Scripted):

[21:52:53] >Plod has left #Chat -- 3 Users -- [email]Blah@192.168.0.rD7=[/email] [234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890

Quit (mIRC default):

[21:55:12] * Quits: Plod (QUIT: [234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890)

Quit (Scripted):

[21:54:48] >Plod has left IRC -- 3 Users -- Quit: [234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890[234567890

Only solution I can see (for the original poster) is to script in a part event and make it look like the default one so it matches the look of other events.