I have driven both of our volts to Las Vegas and back to San Jose six times, a 1000 mile round trip. The journey requires us to transverse 2+ mountain ranges. We do not recharge the Volt during the trip so it is essentially all gas. Sometimes I will drive over the mountains and I will twiddle with mountain mode. Sometimes my wife drives over the mountains and does not twiddle, she just drives.
We have noticed no power loss driving 70+ mph on cruise control over the mountains except when you want to put your foot in it to pass a slow moving "PIUS". It works a lot better when you are in mountain mode and you have the ability to give it a shot of electricity especially at 4000' elevation. So I have gotten in the habit of driving in mountain mode if I am going to consume all of my electricity on every trip.
I think the genius of MM is that if you deplete your traction battery (ie. forget to engage MM when you have the juice), if you engage it 10-20 minutes before you "need" it, it will be charged to the ~20% and be held there.
Needless to say, the cargo space is manageable, like for example, on these long trips we usually take a big ice chest, clothes and food for two, plus my golf clubs.
I'm adding a spare tire to the mix because we probably should have one during our trips through the Mojave desert.
I'm convinced that the cargo space we lose because of the tire, is not that big a deal.
Test drive one on a freeway; put in "Hold" mode, to force it to operate solely on gas. I am sure you will be surprised how much power the car has.