Wow, you're really off to a good start
I googled the
AX Eco and at first glance the only external differences are smooth wheel covers and a short Kammback. There may also be an undertray, but I haven't found any info on that. These aeromods drop the Cd from your AX's 0.31 to 0.28. Weight is just under 700kg. It had the older 1.4 (1361 cm3) diesel, no info as to whether it was tuned. It's fuel consumption of 2.8-3.4 l/100km is what you are already getting, so you are heading in the right direction.
Smooth hubcaps, an undertray, rear wheel skirts, and a Kammback extending to the rear bumper should get your Cd down to 0.28 or even lower. I'm not sure how you can reduce the weight to 700kg
How much can you cover you grille? Is your present grille block the max you can do without overheating? Maybe an openable version for quicker warmup? You need not switch to 165-width tires, Continental makes 155/80R13 EcoContacts, which are standard in the eco versions of the Opel Corsa.