My setup is just like this, but the flames came out the exhaust after letting off the throttle. Never from the wastgate (that I was told). Nice blue flames too, (that I was told) and a couple nices pops. No cat by the way.
Put cat in, stopped the flames.
An old very nice looking chev gets around these parts and must have the switch to do the flames. He quite often drives past us and lets them go. We first saw it on the highway, he heard us talking on the UHF and then put on a show.