On my lupo 2l ecu file there was no boost at all up to 140 km/h speeds as it was not needed. You need only boost when you have to accelerate and stress the engine more to get better efficiency.
It all comes down to air to fuel ratio. In a diesel if your AFR is over 18 you are pretty much burning all fuel you inject. You can inject more diesel to get little bit more power but then you are wasting diesel as not all of it is not burnt.
Why there are turbos then is related to particle emissions as you can control the EGR flow direction if you have higher back pressure than boost pressure is. And yes egr system wastes fuel also but it does reduce some particle emissions.
1.4 SD engine can produce 48 hp and 82Nm, but that engine was used in the 1980s to early 1990s. My 1.2 TDI is a pump duse engine and I can control the injection more accurately and jave higher injection pressures.
if you compare the 1.9 TDI and SDI you need to use its bsfc map:
Take for example reference point from 10 hp line. thats about 2000 rpm in TDI
SDI 10hp@2400rpm-> 400g/kwh->21% efficiency
SDI 10hp@ 2000rpm with TDI gear ratios-> 320g/kwh -> 26,2% efficiency
You would save about 5,2% by installing taller gear ratios from 1.9 TDI to 1.9 SDI.