Hello cuda!
Bravo for the improvement. I had a Megane 3 RS before and I got that sort of fuel efficiency. Pulse and glide made a huge improvement in it.
Do you do short or long trips? Diesel could be a good thing for you in case of long trips.
If you do small trips, or trips with height differences, a lot of acceleration and braking, the main factor for fuel efficiency will be weight.
If you do long, constant and high speed trips, then aerodynamics are the key factor. You need to get a car with the lowest CdA (SCx) possible.
Et vive la France