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I dtoesn't sond ulike you were beating on the car all that much. No faster than a V6 wagon. If the tune and build was correct, it should have handled it just fine.
Take a sample of the coolant to a good shop, and have it tested for byproducts of combustion. That will detect a head gasket leak.
As for handeling, what are your suspension settings front and rear; caster, camber, toe? Those simple adjustments make a huge impact on how the car drives and handles. Once you have those correct, then you can think about improvements like sway bars and better shocks/springs. But you gotta do the basics first, or you'll just compound the problem.
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