I agree with VehiGAZ on just about all the above.
I really only feel any added power when I am passing "at speed". On highways, it gets much easier to pass. Careful, cause once you feel that ability, you will want to do it over and over, heh. Plus, the added bonus of the intake whir and S/C whine with the exhaust rumble will satisfy you.
Backroads and city, I never feel a difference. From a stop, I dont notice any added "oomph". Though, keep in mind, I have bigger meat for wheels and tires so I am trying to turn more with that much more added weight. Someone with stock sized or smaller wheels/tires with the S/C might be able to give you their perception. However, the S/C is a plus to help turn those larger wheels/tires. Without it, it always felt pretty damned sluggish.
As for fuel, I would be hesitant to put in less than 93oct. I had knocking previous to my S/C install and only when I finally installed it and changed over to synthetic, did the knocking go away (mostly). Knocking is one thing, but to let it continue would be murder. So, probably best to follow instructions per the S/C use.
Gary Noonan
'01 S/C VX / '18 Forester XT