manufacturers have traded off one thing for another and I suspect in the 80's 90's were kneejerk jumping at bandaids to magic emmisions lower rather than designing better engines.
I believe the current or later jtdm engines are pretty well designed with as low as practicable compression ratios to give maximum performance and efficiency and still be able to start easily(with glow plug assist)
the whole design seems not to be outdated when reading through technology papers etc, with their shaped piston crowns matching injector spray patterns and as stated multiple injection combustion events.
although personaly im in favour of removing these band aids and assisting the original designers, so egr delete , if I had dpf id delete , cat just put one back in for mot, but exhaust min back pressure, turbo larger at the cost of drivability or spool but gains mpg and efficiency, better intercooler lowers combustion temperature increases air density and so will allow for cleaner more efficient burn, particulates are most likely drastically reduced at the cost of some nox but would need to test to find out.
with the engine derestricted of all the emission add ons the injection timing and pressure could most likely be tweeked to further reduce the nox formation if desired at loss of power and efficiency.
running a more expensive GTL high cetane fuel would even further reduce particulates and nox for fact just google it easy 10% reduction in soot and nox.
with all this in mind and emisions as priority and power a second during a remap id guess that a nicely tuned jtdm would put out equal or less emissions regarding soot and nox and less c02 to a poorly maintained bog standard jtdm with supermarkets worst fuel.
so with just the fuel in mind , how many of the tree huggers who wouldn't remove egr / dpf actually bother to run their cars on the best fuel to save an extra 10% on these nasty emissions at a good few £ per tank difference I bet theres not that many.
im going to google next if my wmi injection helps lower emisions also.
oooh I don't believe the figures but only figures I can find so far
Water/methanol injection has big advantages with emissions. Specifically, NOx is reduced up to 70 percent and particulate matter (PM) is reduced up to 50 percent.
From: Power - Diesel - Water/Methanol Injection Kit - Off-Road Magazine
heres a chart of reduction to get things into perspective as my euro 4 didn't have a pdf anyway
but as can be seen a massive reduction in emissions from the 90s or even euro 1