Actually the MTU power plant used in the Class 172 has rather impressive fuel efficiency.
The primary problem that the new emissions regs is causing for DMUs is not anything to do with fuel consumption, but that power plants with sufficiently high power outputs are becoming difficult to fit into the volume available.
This is why we have not had a class of Turbostars with QSK-19s or similar in it yet.
The latest generation of technologies such as selective catalytic reduction should allow this restriction to be overcome and permit high power diesels to be built which are sufficiently compact. As appears to be the case with the engines for the IEP.
From what I have been told new dmu's in complying with emissions regs, the exhaust system makes them very fuel inefficient. Class 172's very inefficient compared with 150's they replaced.