I have been running the Phantom Grip in my E-Prepared Manta for 1 1/2 years with no problems. I started with the lightest spring (least amount of lock) and have since changed it to the middle spring (more lock). The change to the heavier spring did introduce a little bit of understeer, but greatly reduced the inside wheel sping exiting tight corners.
BTW, the Phantom Grip looks an awful lot like the unit Rally Bob used to sell.
I do not know of anyone using this in a GT. I do know people using them in a street Manta and an FSP autocross car and 1 considering it in a street/track Toyota Celia (RWD). If I still ran a Manta on the street, I would use one.
Rally Bob probably knows of someone using this type of LSD in a GT. You might try posting your questio on the Yahoo Classic Opel Forum.