It definitely works.....when everything is clean, new and working as it should !
It works at part throttle as well as coasting, and the bigger the engine, the better the benefits. Though its primary reason is to NoX by displacing the excess oxygen present in compression ignition engines which have a surfeit of air pretty much all the time (this is the excess air that tuners utilise by increasing injection pulse duration)
Thing is, when things including fuel and oil quality are even slightly off, it throws soot through the induction negating the benefits and making the oil acidic, but this doesn't come out when the cars are type tested for emissions and economy, this is what happens in the real world.
In theory, if you have an EGR and no DPF you are probably filthier than a car no DPF and a deleted EGR, modern engineering, everything you add .....you need to add something else to counter the next effect!