I don't think there is anything special about the rear arms on an STI, they look identical in quality to the 1.6 and unless I'm mistaken are the same part.
What makes the original item good is quality of bushings and ball or rose joints in the arms. The standard sti is same as 1.6 apart from front alloy wishbones as far as I'm aware and the uprated pink sti stuff maybe rose jointed I'm not sure.
The OE are good quality bushings and stand the test of time and I always fit OE bushings when I can on any car but sometimes when separate bushings are not available the cost of the OE arms do not make financial sense (application dependent).
That hardrace stuff could be good and could be dirt I have no experience with them but I wouldn't knock them without trying, if the bushings are good quality then they may be a good purchase