I think SP1 has 4 link rear suspension with a watt's linkage, coil overs and have approximately twice the bump and rebound of the standard rear end. It also cures the problem of the torque, particularly from the 3.7, loading up the rear springs and the resulting movement on the front of the diff.
While developing the suspension the factory fitted it to a roadster.
IMO it is very impressive and looks great with a clever off set for the propeller on the Watt's linkage and, if you are going to have a live axle, this would be the system one would chose.
While many may favour a double wishbones; that system offers some challenges as one needs to locate and secure the diff in the middle of the rear chassis. IMO, without adding considerable weight, there is not the strength in that part of the chassis to do that effectively.
