If your system is pushing coolant into the overflow at lower temps, you've got one of two problems.
#1 Bad radiator cap.
#2 Blown head gasket.
Running a stock radiator on a turbo car is a no-no. Why spend thousands on a turbo setup, then neglect a couple hundred on a good aluminum radiator. Are your turbo's intercooled? If they are water intercooled you are shortening the life of the turbo's by using the crappy stock radiator, along with putting a hurtin on the head gasket's and top end parts.
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