OMC invites understeer in anything, but you can't expect to just dial in the castor and hope it comes to you.

17/24 is a good starting point gear for a 125 but 80" & 125's look fastest when shifting into 6th just before start-finish.  Balance between speed and acceleration depending on your motor.  Acceleration seems to be a bit more important.

This track is a drivers track.  Watch how the fast ones take it.