I know that a 911 is a different animal but that bumper out back on a GTX is way out there. Drag racers also put the battery way out back for weight transfer. That and a steel bumper and brackets will only help getting the pendulum effect going. Moving the battery over the rear axle might be worth the added weight of the cable as it gets the weight of the nose. Or go with a battery that weighs 2 or 3 lbs but not cheap. Run 1 Hi Low horn not 2 as it's out front and cheap to do.

I'm trying to get my GTX under 3400 and most of the weight is from the nose of the car. You can also do some drilling. It's a power window assy. that I took a pound out of ans 1.5 lbs out of the backs.