Its dependent on weight transfer. If the weight on the rear tires is evenly distributed between them then both tires will resist changing speeds an equal amount and the car will want to go straight. If, however, there is a large percentage of weight on just one rear tire then that tire will react more longitudinal force then the opposing rear tire and a yaw moment will be introduced. This affect can lead to power on oversteer during hard cornering with a spool setup.One last thing, wouldn't more lock on corner exit make oversteer worse? (just a question I have personally...Never really looked at diff tuning)
My understanding of it is that when both wheels are accelerating at the same speed (locked), they make the car want to go straight, thus reducing oversteer at corner exit.
In regards to your original question, learning to maintain and tune your current diff would, in my mind, be a much more effective use of your time than redesigning your powertrain to accommodate the MK2. Without experience tuning your suspension and differential setup you won't be able to validate the effect of changing your differential. Redesigning everything for the new diff will also require more resources during the design phase, which won't help you finish the car sooner and without validation you will be right back where you are currently in a few years.
That's my two cents. To tell you a little of my experience, in 2012 I redesigned my team's drivetrain to work with a Drexler after having used a Torsen for over 10 years. My motivation was purely for reliability and finishing the car sooner and we never really looked at how the diff affected the car. Our drivers haven't noticed a significant difference, but this summer we are looking at doing some diff testing to validate its effects. Unfortunately its difficult to change the setup and because we don't now very much about it the diff testing is very low on our priorities list.