I think I will givethose bosh plugs a try aswell ifyou say that they are that good

 

If you know a good mechanic, maybe have him hook up a scanner and watch your O2 sensor voltages, o2 switching rates, long and short term fuel trim numbers, engine coolant temp reading, MAP sensor readings, intake air temp readings, all of which will be on the datastream.
Any issues may cause lowered fuel mileage without necessarily lighting your check engine light.