2.) You will experience smoother performance as dirty hot air is no longer being pumped into the intake manifold, stupid idea really, as you want nice cold clean air for combustion.
It's done for emissions. At light load, you recirculate exhaust for the following reasons:
1. It bulks up the hot gas in the cylinder to reduce the max temp, reducing NOx production. NOx (Oxides of nitrogen) gives cities that brown haze on sunny days (the sunlight reacts with it).
2. It slightly helps the valves as they are not dealing with quite so hot exhaust. The total energy is the same as you have more cooler gas rather than a little hot gas (re Boyles law)
3. It slightly reduces throttling losses, as at light loads you're pulling a partial vacuum on the inlet which is a waste of energy (a marginal benefit).
At full power the valve has to shut as you want max clean air to burn with the fuel.
BTW, diesels have more problems with NOx as they are un-throttled engines so always burn lean at light load - the high residual oxygen at light loads produces much more NOx. A diesel's EGR valve is to reduce the oxygen content at light load. Again, obviously at full load you want it shut as you want all the oxygen possible for power, and the NOx problem reduces as the oxygen is used on the fuel, not post-burn oxidation of nitrogen. If you frig out a diesels EGR it'll prob throw the engine light as its a no-no. I thought it would on a petrol as well, but evidently not.
