A divorced wastegate has a plate the protrudes into the turbo to separate exhaust flow coming out of the turbo to give reduce turbulence. This is good for stock and smaller turbochargers the protrusion does not work with some aftermarket turbos like APS turbo’s for example. A Bellmouth has a wide open design that is better for larger turbochargers with more flow. It will work fine with a stock or small turbo, but it is designed for scenarios where wastegate turbulence is not an issue. Invidia’s catless downpipe will cause a Check Engine Light on your WRX/STi.