I would do a good compression test which could indicate if there are some valve related issues. The recommendation to check the fuel pressure is a good one. Pull some of the spark plugs and check for the presence of black soot or wetness. It is possible the coils are underperforming in terms of their discharge energy and need to be replaced. I did this on my 3.5L Benz and it made a measurable differenence in fuel consumption and engine performance. Maybe the resistance of sparkplug wires is excessive and need to be replaced. This thought might be on the crazy side but could the odometer be faulty?