I had a brief discussion about the Giken with an SRT engineer at VOI 12. I told him that I changed out a Gen IV diff after the Gen IV posi traction crapped out because the Giken is far more robust. I also said that I thought it was a far superior piece and that my Viper tech and all the mechanics in the shop were very impressed with the way it was put together. My guess would be that not using it may be related to cost.
Another product you should convince SRT to use as a standard along with MCS....