As per my experience, a 660cc Turbocharged engine is around 4k RPM at 100 kph. This is too much work for a small engine. The economy you are getting is reasonable and I doubt that you will be able to improve upon it.
Try driving a little slower, like around 80-90 kph, you will see that the economy improves drastically because the RPM will go down. Higher RPM on small engine can be more fuel consuming as compared to low RPM on bigger engine, as there is lesser work to do.
