That's what I've heard as well, the £5 jobbies are simply a resistor in the temp sensor wiring to fool the ECU into thinking the engine is cooler than it actually is. This will make it run rich ALL the time with the exception of light throttle cruising and at idle when the Lambda sensor is doing it's thing. Yes, pick up MAY be quicker but long term you're looking at carbon build up in the combustion chambers and a dead cat. So what's that, £200 for a top end de-coke and £150 for the cat? Hardly a bargain if you intend keeping the car for a while.
I wouldn't touch them, but that's just my opinion obviously.