apparently the mrt cat's had problems which they are now fixing. many people have ceramic coated the headers of the full mrt system which has fixed the problem. you may want to try doing this, but since you are converting the car to stock, just fix it and sell it.
mrt now ceramic coats the headers from the factory to fix this problem. you may want to e-mail them about the problem as you are not the only person that has had this problem.