OK, in simpler terms what are the upsides and downsides of the blanking plates. Is using two the way to go or just one? .
Most people just have the one plate, the diamond shaped plate is probably easier to fit (literally two bolts). I would say you get slightly better improvements with two plates. There are a few variations on the diamond plate, one with one 8-9mm hole, and one with three 5.5mm holes (there is no consensus on which is better, but Alfa sell the one with three holes in (see the bulletin halfway down this page: http://www.alfaowner.com/Forum/alfa-...ing-plate.html
Remember the only reason to fit an EGR is to hit certain emissions standards, where these standards are not in place, manufacturers do not fit them.
Upsides to no change:
The car is as original and has lower Nox
The Car warms up slightly quicker (though most don't seem to notice)
That's about it
Without a map, a blanked EGR would throw up a light on the dash (but would run normally/better), a restricted EGR would not show this light, but gains not as good.
Upsides to blanking/ restricting
Car responds quicker at low revs
Lower particulate production (which would normally fill up the Dpf quicker)
Less soot blocking up the manifold and going into the cylinders (reducing car performance over time), obstructing swirl valves, reducing flow, etc (even pro EGR fans will admit this)
Fewer problems with the EGR (often get blocked open)