The restrictor goes on the inlet side of the turbo within 50mm of the blades. Depending on the model of GC8 (and turbo) you chose, will depend on how the restrictor is fitted.
The TD turbos require a bit more work to fit the restrictor than the VF's.
It's not an overly complicated job on either turbo and the standard hoses can be reconnected once the restrictor is in place.
You can run a standard
ECU with a restrictor and the car will run fine, it just won't produce as much power. Many people are running the standard
ECU with a 34mm restrictor in gravel rallying at the moment. It's a cheaper way to get started.
The best method however is to use an aftermarket
ECU/chip that can be mapped to gain the best power and torque available with the restrictor in place.
Restrictors were imposed (at least for rallying) after the spectacular and highly dangerous Group B era in the
WRC. It took a couple of years before the replacement Group A cars started to have rising power outputs. In an effort to stop Group A cars reaching the outputs of Group B, the FIA introduced inlet air restrictors for forced induction cars.
After many years CAMS has recently allowed a change in Australian Rallying to remove restrictors from turbo cars competing at national level (ARC).
Not sure on the race front, I'd say the restrictor is in place to keep some form of parity amongst the turbo cars.
Karl