I haven't found a car that keep hang with my 68 on moody lane here its a long hilly curvey country road here.Theres not many cars that handle good to me some let you think it but they don't.

 

I know you said your car already corners well, but maybe replace the upper and lower control arms with tubular ones to go with the springs/shocks you're already running. They're stronger and won't flex in hard cornering. Add this to some subframes, export brace, a monte carlo bar, 1+ inch front rollbar, and a rear panhard barand you should be really solid.
Other than that it's hard to say without more info on what you got on there already. Disc brakes all around are a must, maybe a T5 swap or add some power to the motor.

 

What lexus do u have?i have an 02 IS300 and it handles pretty well..just to add the mustang has 4.5 mideyes,620 with a coil cut, front disk,dual res,302 roller block,c4.thanks for the input