Does anybody here know how efficient the current scheduling is already,

Mnemosyne uses one of the older Supermemo SRS algorithms; Peter (the Mnemosyne dev) has said in the past that the more recent & complicated Supermemo algorithms offer little advantage.

or how much differences there are between different people/topics/whatever?

I dunno. The relevant statistic is probably 'easiness'. IIRC, there is in fact a deck-wide easiness factor that is slowly adjusted to deal with users who systematically make very slowly or very quickly learned cards. You could try looking at the torrent of Mnemosyne statistics and see how widely that statistic varies from person to person?