My brother was travelling cross country and suddenly needed a new catalytic converter on his '87 Camry. A small town mechanic gutted it to keep it running until he could get home. He said he had a lot more power and 2-3 mpg better (I guess that is 10%!) performance. Down side is that it becomes a bigger polluter, and it was no longer legal to drive for that reason. He is environmentally conscious and paid something like 1200 to fix it.
My reply to Ice's question. I like the new civic a lot, but want something larger, and that can run on electric alone rather than in parallel. My hope is to upgrade to plug-in lithium Ion batteries and eventually avoid gassing up almost completely. A local MA company, A123, already has upgrade kit for Prius. I have had my last car 15 years and expect to keep the next one almost as long. Who knows what gas will be at in 2023?!?