This has always been my opinion as well and I've often stated that the diff in this car is quite small and underbuilt for what it is asked to do. It is this thought, that I've considered fluid changes at a more frequent rate to be necessary if the car is used "vigorously".
I think the amount of your change intervals should be more than adequate to safeguard your diff. The fact that it hasn't certainly does bear witness to what you've said.

 

Let's be clear: it's fairly rare, I think, that the actual Torsen unit fails; it's the ring & pinion gears that are the weak point.

And it isn't always "abuse"; anyone who racks up a few thousand track miles on R-compounds is going to have issues, no matter how careful they are with fluids.