BMW 318i 2007

Expensive cars are built and repaired with expensive parts. Not sure where you're located, but in the U.S. it is illegal to remove catalytic converters. Also, their efficiency is monitored by the Engine Computer while you drive. When no change in the composition of the exhaust gas takes place, the computer will set a diagnostic fault code in memory and turn on the Check Engine light. If you drive with that light always on, you will never know when the computer has detected another problem. Also, there are conditions that must be met for certain codes to set and one of them is that specific other codes can't already be in memory. Your computer will not run some self-tests and won't be able to make adjustments to the fuel metering. That means you will be polluting excessively AND wasting fuel.

In the U.S. most emissions system components have a 50,000 mile / 5-year warranty. You might inquire if your converter still is under such a warranty.