Unless someone go up a very very long hill and completely discharge the battery to the lowest point then at that point the power that drive the car should come purely from engine which it has to run at high rev to cope of load. Not sure how high it have to be which depends on size of the engine, Motor Rated Power, Speed. If drive up at slower speed, it may not have rev up to high rev that has annoying noise.

Sometime people forget what they might want. I would take 80-20 rule, if Volt can meet 80% of the population who might buy them without rev up very high rpm. Or if Volt can run without rev up very high rpm for 80% of the time people use them then why would someone worry? I even believe that it should be 99-1 rule rather than 80-20 here. Do climb the long hill at least one day in the last 3 months (100 days)? I can be wrong but I would guess 99% of the people don’t. Why would any company design the car just to satisfy the last 1% and waste lots of money when rent a car which is best at climbing the mountain is a lot easier? For me, I am ok if the car have to rev up very high once in a while or if I have to go up the hill somewhat slower when the car consume significantly less than others.

If someone want to feel better to hear the engine noise and vibration, a small electronics circuit would be able to simulate that for you, I guess:-)