1991 Cadillac Deville car lacks power

The engine will run normally but you'll have excessive emissions. The Engine Computer will still aim for the best fuel / air mixture but the excessive emissions that still occur won't be cleaned up.

You won't get away with removing the converter on a '96 or newer model because its operation and efficiency is monitored by the Engine Computer which will set a diagnostic fault code and turn on the Check Engine light. Besides the increased emissions, you would never know when a different problem is detected, which could be a minor one that turns serious, because that light is already on. Some self-tests won't run either so some causes of running problems won't be detected. You'll never solve those until you reinstall the converter, then let the computer detect the cause of the problem.