See I disagree!
Having spent most of my life driving cars before cat converters were invented and have one car that doesnt have a cat I believe it is most beneficial to remove the cat.
I remember when they came in and either cars engines were upgraded significantly or the power dropped.
Now there are problems, its unlikely you will pass the MOT emmissions test with the decat fitted. The solution will be to keep your cat and swap it before MOT time.
Now as to their effectiveness - no idea really!
Just putting one on a standard car will probably have little or no effect.
With some mods to increase airflow there should be an improvement.
