Mercury OPTIMAX 200 problem need help

Hi, I have a problem with my mercury OPTIMAX 200 engine, year 2006. Engine has lost rpm, it usually runs between 5300 and 5400 Rpm at wot. Last few times I went out with it Engine maximum rpm is around 4200rpm, I have replaced spark plugs, fuel filters but problem remains. It also seems to get worse when I leave it on low revs for a while and then try to open throttle maximum rpm goes down even more to around 3400 rpm. Today I got my mechanic, he connected the computer and found no faults, he replaced some high tension leads and coils but still problem persist. One thing he noticed is when he hooked up gauges to check fuel and air pressure, pressures are ok both for air and fuel with ignition on but as soon as I turn ignition off fuel pressure drops down strait away to zero air pressure remains good. Any suggestions? Engine sound perfect on idle. Last thing I noticed is that engine is using nearly double litres per hour at 3500 rpm then usual. Thanks in advance for any suggestions

Another 2 things I forgot to mention is that engine seems to be making a bit more smoke then usual, and no petrol is visible on the sea surface. Any ideas please as in lost. Thanks