1.Q. "will not excess fuel simply be returned to the left tank no matter how high I set fuel flow?"
A. No, each injector nozzle has a fixed orifice. Fuel flow is what goes to the fuel distribution valve, then to the six cylinder injector nozzles. All of the displayed fuel flow is consumed by the engine.
2.Q. "why adjust max fuel flow? Are there other considerations that I am overlooking"
A. Yes, the mixture is directly affected. For a given throttle setting and RPM, the volume of air is defined. The fuel flow then, determines the mixture. Too lean, reduced power. Too rich, reduced power as well as combustion deposits that mess up the innards of the engine. Perfect mixture for maximum power just gets the CHT too high and shortens engine life quite drastically. It seems that for higher powered engines, about 10 horsepower per GPH is about right.
3.Q. "I just don't understand why I'm being advised to set fuel flow at 30 gallons on my IO550-B"
A. For a 300 hp engine, 30 GPH is going to give the best compromise of engine life and takeoff power.
4.Q. "Is it possible to have too much fuel flow?"
A. Yes. Too high a SLTOFF (SeaLevelTakeOffFuelFlow) the power will fall off, the intake valves and top piston ring grooves will cake with carbon deposits, oil will get dirtier, greater amounts of lead will accumulate in the crankshaft and other internal engine parts. I am not even going to mention the cost of fuel!
5.Q. "Last question...at what point in the fuel system is excess fuel returned to the left tank?"
A. The excess fuel is taken from the top of the swirl chamber of the engine driven mechanical fuel pump. From the cockpit selector valve fuel, goes to the fuel pump inlet. The fuel is then metered, at the fuel metering unit, by the sum of the position of the throttle and mixture controls. The fuel seperates and some goes directly to the fuel distribution valve to be burned. The fuel not passed by the fuel metering unit goes back to the fuel pump. The fuel flow that you see in the cockpit gauge is the fuel that is actually sent to the cylinders. It is adjusted by a needle valve that restricts the fuel returned to the tank.
Hope that helps.