Oh and there is bad batch of xud9te engines, it was something from 97- onwards that you better steer clear from.
9te can be very good for MPG, 3.6l/100km two tank average with 200kg load and wagon ZX body is my best result from it. 309 being lighter and more aero in general shape, it should be even better, whatever that is in MPG, I'm sure that it is better than what you are currently getting
Turbo models can get better mpg than non turbos too, it has to do with BSFC maps, diagrams and clever things that make one's head spin, but to put it simple, more air is more economy with diesel, as with diesel you are always running lean and at low power demand with turbo you can go even more lean than with NA motor. There is always a limit where there is not enough power coming out, but at steady speed you don't need much power and no throttle butterfly means that you can get boost with very little fuel as long as air is moving enough to overcome internal resistances and so on.
If one could decrease displacement of 9te and put silly turbo and lot of boost to make enough power when needed, then it would be even more economical.
If you open boot a bit and duct tape it to such position that angle is no more than 20 degrees, then you would gain few MPG an maybe 10Mph top speed, but it might be already bit extreme
Your lowering is getting better MPG too, but you really need to lower rear too to get that improvement as raised rear actually causes now more drag than original position.
My next target is 3l/100km, but it might take a while as at the moment I don't have even single car on road.