De-cats are basically a pipe to replace the cats, this removes the restrictive flow in the exhaust, some cars benefit from this and some don't.
It does sometimes have a trade off in that it can reduce torque at the lower end.
Pardon me for being ignorant or pedantic, but does removing the cats and replacing them with straight pipes not mean that the car is spewing out untreated gases into the atmosphere? If so, you should all be ashamed of yourselves.
Also, is it not illegal? Would the car fail the MOT?