Short of a flatbed, it's virtually impossible to take a car equipped with the Sentry Key system. Of course, with a flatbed, it goes. Then your only hope is a lojack type system.
Dont bother wasting your money on immobilizers, you have one of the best immoblilzer systems on the planet already, the SKIS. Your only concern is tracking a flatbed with your car on it. I know Lojack is supposed to rock where it works, in urban areas. For me, that would not work. A member of the SRTforums fronts for a company called vehiclelocate.net
I know Dale Seeley used it in his 2005 One Lap of America car, the entire were able to see his progress real time. Road speed, lat/long/ the whole deal. Seemed pretty cool, its a GPS based system.