The deeper the shaft, and hence the exhaust, the more important it becomes to vent the exhause relief outlets obove the wate line. The deeper shaft increases the back pressure throught the exhaust outlet in the lower unit, particularly when the motor is idling or at low speeds. This will therefore increase the amount of exhaust that exits via the pressure relief holes (so there is no back pressure, regardless of the level of the lower unit exhaust outlet). When operating at higher speeds the through prop exhaust sucks the exhaust out (or is designed to do so - but perhaps not completely) A prop/exhaust that is one foot lower than a shorter version increases the back pressure by 62 lbs/inch.
But still I would opt for a longer prop shaft -and in no case take a standard length because it pops out of the water - in my experience. Other than as mentioned above, I don't see any disadvantage to as long a shaft as you can get.
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