Yeh, real accurate results there haha . I agree that it would not be in the order of 1 psi difference... It is only a 1-2 m section of exhaust with half an inch difference...

The cat and number of elbows will have the largest effect on back pressure, if the diff back is only section you are changing the diameter.

The most important message here is that it has nothing to do with "maintaining the gas velocity as the exhaust gas cools". There are so many fundamental things wrong with that statement, bernouli would be turning in his grave.