Some estate cars have a 'softer' rear end body structure than the corresponding saloon. However, always comparing model for model with the same engine, the Insignia 2.0 CDTi estate has the same maximum permissible towload (1600kg) as the saloon, so there is no reason to suppose that the estate is at a disadvantage here.
The lowered suspension makes no difference as it is harder, too. It is only lower when not fully laden, but at max. load the ride height is the same as its non-sports counterpart. Therefore, there is no difference between towbars whether the car has sports suspension or not. Even the 4x4 Insignia has the same towbar.