Hi again Pengze,
You have had a busy weekend posting questions.
1) & 2) You should have records from the Test Bed and Sea Trials of what the Exhaust back Pressure was originally, and the maximum permitted back pressure should be detailed in the Instruction Manuals, along with the diameters of exhaust pipes required and the additional diameter required as the length and number of bends in the exhaust pipe increase. If not email the maker's Service Engineers and they will tell you what it should be for your specific engine. There will inevitably be a small increase due to fouling, over time, as Jimmy says, but this should be small, and the original Sea Trials reading should be well below that in order to allow for later fouling. I would guess within the good old Engineers allowance of 10% for everything!
3) Fouling of the TURBINE causes it to develop more power, running at higher speed. and delivering more scavenge pressure while the efficiency and output of the engine will be reduced. Any Basic Marine Engineering Text Book will confirm this. As I said before fouling reduces the area between the blades through which the gas flows, therefore, the gas velocity has to increas in order for the exhaust gas to get away from the engine. This causes the pressure between the Engine and Turbocharger to rise, reducing the engine power output. At the same time the increase in gas velocity gives it more Kinetic Energy and Momentum which the turbo Charger converts into mechanical power increasing the TC RPM and hence the scavenge air pressure.
4) The Manometer at the Turbo Charger inlet is usuaally connected across the intake filter and is there to tell you when the air filter needs to be cleaned. It doesnt tell you anything about the efficiency of the T/C only that a dirty filter is restricting the air flow.
It is always better to ask a stupid question than to do a stupid thing.