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Oh, intake?
To make 6 psi on an average-sized four-banger takes about 10-15 horsepower worth of power, going off of what Thomas Knight did with their electric superchargers back in the day.
Normal operating vacuum wouldn't even be able to turn such a thing, let alone produce meaningful power from it. If you've ever dismantled an air intake and put your hand over the intake hose or carb ("burping" the carb to clean it used to be such fun!), you'll know how little vacuum it produces at idle or low rpm... much less than a shop vacuum. Now those vacuum motors have the equivalent of about 2-4 horsepower... So say you have about 1 hp of suck available at cruise... then consider some of that suck has to go to intake air to power the engine... you're getting much less than that going to the intake alternator.
Consider it takes at least 2 hp to turn a regular alternator, and 10-20 hp at full electrical load, there isn't much of a comparison there.
More effective to run a generator off the exhaust side. Cap the turbine speed by braking it down with a generator when at full load... also eliminates the need for a wastegate, as you can control maximum boost pressure, as well.
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