RE: (Forced) Induction System Options
I'll take a shot at this.
1) If you turn a fan off, the acceleration of air from the fan ceases, but air still flows through it. I'd assume a supercharger is much the same in it's various incarnations.
2) As the turbo spins, it increases the air/gas pressure through compression. Therein, it's taking in more air and fuel in the same amount of time that it would if it were running naturally aspirated. Increase the air/fuel flow, you increase fuel consumption. The rate increase is not direct, but more exponential because of the change in programming to get the air/fuel mixture correct under higher pressures and at higher rpms. Additionally, when the wastegate opens (once maximum predetermined pressure is reached), the system is theoretically less efficient since more intake is being produced than the engine requires/can handle.
On a side note, *** from a friend who has been running and building dragracing cars for many years, he indicates that Superchargers tend to "oval out the cylinders" after a while. He said this happens at differing rates depending on the heads on each motor. His experience is that the cars with intake valves set closest to dead center of the cylinder/top of the piston usually hold up best because the pressure from the SC is not coming into the cylinder from such an angle to the top of the piston, reducing the side pressure and not pushing the pistons away from the motor's center. He said this tends to collapse the rings or force them into the cylinder walls, causing more wear, friction and heat. The three combined shorten the life of the block.
***This friend has been drag racing since the 60's against the best local talent (over all of these years) in sanctioned and "other" events. He still build cars - he's currently working on a 38 Chevy which dyno'ed 980HP - and raced against Shirley Muldowney when she lived and raced in this area.