Justin,
Thank you for your 5 Star Review and added pic of the GReddy Revolution RS Cat-Back Exhaust for the BRZ/FRS! We are glad to hear that you're impressed with the quality of the welds and the overall fit of the exhaust. It definitely adds plenty of sound and looks amazing!
Regarding the issue you mentioned with the stock diffuser, it's not uncommon for aftermarket exhaust systems to create more heat than the stock system, which can sometimes cause melting or warping of plastic components. This is usually not a major concern as long as the plastic is not melting too severely or causing any other issues.
Regards,