Just to clarify Joel, To my knowledge the 12 and 20Kg/m were not STi and STi/Ra default fitment. To assume that any STi or Ra automatically has one would be incorrect.
I've never found anything other than a boring old 4kg/m softie in any road going non dccd STi. If the car was rallied or 'dirt trial' raced in Japan you may be lucky enough to find one fitted.

I've tried all of them.
Stock 4 kg needs help to stop excessive front wheel spin.
12 is my pick as better than the rest. Does the job nicely, no nasty side efects.
20 behaves like a 12 but has noticable bind on tight turns.

My old series 1 Legacy had approx 220 horsepower at the most. A 20kg/m may have been more beneficial if the car was more powerful. I now have a DCCD centre in there and I'm seriously considering going back to a 12. The DCCD is less predictable.
I did loads of *handbrake turns with the 12 and the 20, so don't let anyone tell you it can't be done. *must have a hydraulic handbrake, plate rear diff also desireable.