If it helps back in Oct 2005 a couple months after I bought my TTZ which was/is stock which needs help right now which is another story. Coming back from Orlando going on the Turnpike I drove about 154MPH it wouldnt go any further becuase of the limiter. Car Was stable, felt solid. There was no vibrations in the steering wheel at all. I was surprised because I'm using to driving in cars that shimmy back and forth soon as u try to top it out. Only bad thing was the Right Rear Tire Blew at 154MPH !!! After pumping the brakes and gradually coming to a stop I came back to reality. Tires were 2 years old and I was still learning about cars at that time. I feel with these things it will be certainly stable: 1.)New Tires Preferably Z Rated and Balanced 2.)Alignment 3.)Independant Suspension in proper working order w/HICAS disabled 4.)Smooth road surface 5.)Rear Diffuser 6.)Possibly An aero underbody panel These things in my mind will gaurantee stability