When I talked to Tony a couple months ago he mentioned he felt the Onas needed a bit more vertical fin area, so I'm guessing this might be more of a 'while I'm making changes' sort of thing rather than purely as a result of the contra-drive. By increasing the vertical fin area, he's increased the static yaw stability, requiring more rudder surface deflection for a given response.

Removing spiral slipstream and the resulting required right thrust could theoretically also 'unload' the fuselage making it feel a bit more 'loose' in yaw around neutral. I'm probably not explaining very well what I'm trying to say. The spiral slipstream applies a force moving the tail right. The right thrust counters this force, and the two forces working against each other creates almost an aerodynamic inertial effect making the airplane more resistant to yaw changes around neutral.

Sorry if I'm rambling...