A K&N filter will give you a little better performance and consumption compared to to a standard filter, and it's re-useable!
To calculate the consumption you need to use the actual amount of fuel used (that is, what it shows on the fuel bowser after filling)
then divide that by the actual number of kilometres travelled.
Your figures of 19l/100km mean that the 57 litre tank was bone dry when you filled it after doing 300km...........
If you do 320km and the pump shows 49 litres, that's 49/320=15.3l/100km.
Still excessive, but nowhere near what you describe.
You haven't mentioned the condition of the tyres, wheel alignment, etc.
These factors all influence consumption.
Also, based on what I can read about the history of the car my guess is that the plugs & leads are still the original,
as are possibly the tyres (if they're Goodyear NCT they're crap), and will cost you at least 2l/100km.
As I said, 2.4l Auto and driving with the air-con on...... don't expect much better than 14l/100km in the city,
and that also depends on how much weight the car is carrying.
An automatic car, with the air-conditioning on, in traffic, will always use a lot of fuel.