Always pretty dubious about the claims of doing these types of things, but actually it has dramatically reduced the amount of black smoke when booting it, its almost nothing now
EGR - no doubt about it, feeding exhaust INTO the engine is not good, clean air is, nice clean diesel burn now.
We have a VW Jetta in our fleet and when I'm driving it I pull the vac line off the EGR and plug it with something, usually a .22 case it tells me the emissions system has a fault and to go to a garage immediately (really?) but I then drive it and get 8-10mpg more! That is without a word of a lie. I then plug it back in and the light goes off within 5 miles.
10mpg more, that's quite significant just under 20% better, well I tell you less fuel for more miles EGR or not is go to be better for the environment.
Friend of mine bought a new tractor about 3 years ago, it was the latest Euro spec engine, used move diesel than his older tractors and that's progress!
Google EGR and California, where it all stemmed from - Californians are hypocrites - they want all their big engined cars but moan about the pollution.
By the way did you clean out the intake manifold, mine was down about a good 30% in diameter with soot.