Hey Charlie, Thanks for sharing your experience with adding weight to the rear of your car, because me being a spec/amature driver, I would have never felt the difference!
Porsche built a fantastic car by placing the tranny and gas tank where they did. By adding weight to the spare tire well, is going to add nearly as much weight to the rear wheels as it's going to take weight away from the front wheels. Driving a neutral car is important, but setting it up for maximum gforce is also important, so I believe the physics needed to accomplish this would be adding weight between the front and rear wheels to maximize grip on both ends as opposed to increasing grip on one end and decreasing grip on the other.
My personal opinion on a well set up car especially for an amature or spec driver is that it can decrease the odds of loosing control, spinning out or what ever and can also enhance the learning curve with the confidence that a well balanced car gives. I know in AZ we have guys passing in places that they didn't before because of the confidence gained by having better traction from the RR's which is similar to having a well balanced car.
I know the easiest and shortest answer to a set up question is "it doesn't matter with your driving talent" but it wasn't that long ago that I was asking those same questions and got pretty fast by learning all that I could about set up coupled with seat time, lots of seat time!
Charlie, I sure do hope that you will place a cinder block in your spare tire well and detune your car to 128hp for nationals to prove your point