I think IRCNet supports topics up to 160 characters (at least the server says "TOPICLEN=160 " when I connect), but mIRC shows only 80 of them and the rest disappears. Is there any way I can change that limit? I've spent several hours trying to find a solution...

dunno if its the case here, but remember that bold, underline, colors etc also count as signs - the server doesnt see it as a control code, but just as signs aswell, so for example a topic like "14This is my 15Topic" is actual 27 character, though it apears to be 16 character to your eye.

to test it:

No, topic is normal text, there is no specialities.