Jacking the rear end with elongated shackles will cause the car to become tail happy, or in other words more prone to over steer under braking and acceleration. This condition is not desired in a street car where it becomes unstable under hard braking in traffic.
If you are now thinking leaving rear at factory height, and lowering front from factory ride height by cranking down the torsion bars, that will make for crappy handling and ride as well.