You said "That reduced soot is due to a leaner mixture (makes sense with the fuel savings)."I'm not sure what you mean by "lean it out" in this context. I think you misread something I wrote.
And what I'm saying is when not being overfueled (or under low boost acceleration) a diesel always runs lean. There will always be excess air. It's simply a diesel fundamental. If you try and make it run leaner than it normally would then you just lose power or RPMs because the engine will seek equilibrium for lack of a better term. Yes, the VGT/VNT turbos help control boost better under a wide range of conditions, but you are not going to get a diesel to run even leaner while maintaining power/RPM. Fuel controls everything. Doesn't matter that there's a VGT, that relies on combustion. Increase boost without increasing fuel and you'll lose power.
Perhaps a better statement would've been reduced overfueling rather than a "leaner mixture" as the latter implies the engine is running leaner than its already-lean normal operation. If I lose weight but am still overweight then I'm not skinny or lean. I'm just less fat. Reducing fat mixtures to reduce soot isn't the same as running leaner, and while "leaner mixture" isn't technically wrong the wording has implications that don't really fit what's being done to control soot.
Less-rich/fat vs leaner...Semantics? Perhaps...but sometimes that makes all the difference in making sure your point is getting across clearly.