I am not familiar with those flame retardants, but you can identify the poential effect by DSC testing to see if there's any change of the melting enthalpy. Flame retardants may paly the role of nucleating agents. The degress of crystallization has obvious impact on the mechanical properties, such as improving modulus, tensile strength, but may compromise the impact strength and enlongation at break if the degree of crystallization is too high and the size of the crystals are too big.