What is better, to remove the stock cat or to fit a metallic sports cat in its place?
The correct choice depends on the use of the car and the owner preferences.
The removal of the cat results in a very free flowing exhaust system, with low backpressure. This is positive only if the electronic management is able to put this freedom to use.
The ECU reprogramming is advisable with this modification. On many cars this procedure allow the deactivation of the second O2 sensor, the one after the cat, so the incorrect values will be ignored and there won’t be any warning light. This is standard procedure on race cars with stand alone ECU.
The downside of this modification is that the car won’t pass any emission test as the pollutant gasses won’t be abated. An unpleasant odor can also be perceived from the exhaust.
A metallic sports cat offers a much higher gas flow than a stock ceramic one, because of the much lower cell density, 100 or 200 cells per inch square compared to the 400-800 of the stock unit. At the same time it retains some backpressure compared to the no cat option. This makes the ECU reprogramming unnecessary in most cases, but it still allows a consistent performance increase thanks to the ability to adapt of the modern electronic management systems.
There won’t be any problem with emission tests and there won’t be any unpleasant odor.
It must be pointed out that on very recent cars (Euro 4 and 5 in European standard) the emission range set by the manufacturer is so restrictive that even the HJS 1st grade cats used in the Supersprint exhaust system are not able to avoid the emissions control warning light.