I drive my 400E pretty aggressively, I often shift the transmission by myself and heat 80kmph in first gear.
We use winter gas in Canada, my gas mileage is now 12.5L/100km (18.8 mpg) for combined highway/city stop and go traffic. If I take it for a long highway ride, and keep the speed at 120-140 kmph, I normally get below 10L/100km (23.5 mpg). It does better than this in summer.
9 mpg is definitely unacceptable unless you drive on a race track and keep your engine rpm above 4000 all the time. Like other has suggested, O2 Sensor should be checked, I would guess somebody can see black smoke from your tail pipe with that gas mileage.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.