The effect on my TD5 Discovery was very good, and it now goes better, and (last full tankful) does just over 32 miles per gallon.
Just after I fitted the EGR plate to the Disco, I fitted another to a farmer chum's TD5 Defender. He phoned me from the Defender next day to say he had a trailer full of cattle on the back and the Defender was romping up a hill in FIFTH gear it used to struggle on in THIRD! He still says it is like having a second turbo. He can leave other TD5 Defenders dead on the road.
The Defender engine is set up differently from the Discovery, and it does seem that blanking the EGR (at the exhaust manifold end is best) makes a really useful difference to the way it goes, and the fuel consumption.
For fiver, how could it not be a good plan?
Charles.