What happens when you approach Vmca?
The aircraft starts to roll and yaw toward the failed engine as you run out of aileron authority.
The initial tendancy is for the aircraft to;
1) Yaw to the failed engine due to asymmetric thrust,
2) Yaw to the failed engine due to asymmetric drag,
3) Roll to the failed engine due to yaw in that direction,
4) Roll to the failed engine due to asymmetric lift (airflow from the live engine over the wing and nacelle),
5) Yaw to the failed engine due to increased drag produced by the downward deflected aileron on that side trying to counter the roll,
6) Slip generating roll and yaw toward the failed engine.
All these effects need control inputs to arrest the undesired changes in attitude happening.
The slower you go (ie. approaching Vmca) the less control effectiveness available, the more deflection required to achieve the same effect and the sooner you reach the physical limit of deflection.
Vmca is a control speed.
Now if you increase the yaw toward the failed engine you are going to reach the limit of aileron travel earlier to stop the aircraft rolling into the turn (which it already wants to do). More control deflection means more drag and any increase in drag is going to be detrimental to performance.
If you need proof of this just get your instructor to simulate an EFATO and do a circuit turning each way. You may not even maintain height turning towards the dead engine. It doesn't matter if the props are CR or not. There WILL be a noticeable difference.
Addenum,
The critical engine is specified as worst case scenario. It's failure will create the greatest problem and may not be prop direction related. Aztrucks may have CR props, but early models with the hydraulic pump (for gear and flap actuation) only on the left engine makes it a left engine critical aircraft.
Simply speaking, picking up the dead side by 5deg bank and 1/2 a skid ball to the live side is a compromise between using all bank (fully balanced- max rudder) and wings level (not balanced- max aileron). It allows for the best remaining control deflection in comparison with the best drag reduction (basically). It is also the max specified bank allowed in the determination of Vmca (or Vmc as the FAA calls it) as per FAR 23.