For freeway driving, all driving was cruise control at the speed limit. For mountain driving it was sane turns, and not above speed limit. Climate control was 8% (fan only) during the climbs. Temps ranged from 60 in the hills to 78 on S2.
Dropped into CS mode just before I got to the 1000' mark on the climb out, and just before hitting 4000' for the first time after about 17 miles (4% average grade on the slope part, but "lumpy") the "reduced power" warning sounded. BUT! Right after that there's a small valley before the first crest, then a big valley.
Car did fine, and had no trouble maintaining 70 MPH. The warning went away after the first crest, and that drop to 3500' at Diablo canyon. It didn't come on again up to the Sunrise Highway turnoff at 4100'. In fact the engine fully idled down at the stop sign at the end of the ramp (I had a wait for a Prius coming off the mountain to turn across me, driver and passenger STARING at the Volt as they went by :-).
The 2000' mountain climb up Laguna at 35-55 MPH over 8 miles (5% grade) had no trouble. Engine stayed at normal max RPMs the entire climb, and having reset a trip odo at the bottom observed 15 MPG for the duration. The little 1.4L can suck gas!
The climb back up the other side of the hill, from the S2/I8 intersection in Ocotillo west was a different story. Starting at 400', there's a direct climb of ~3000' straight up over 13 miles @65 MPH (4-5% grade, with no "lumps"). Just before making the first peak the warning came on, and sure enough speed started to drop. Came down to 60 (which actually was the limit in that turn) as I rounded the top, and stayed on as I got to the flat and the first valley.
However, it let me accelerate to the new 70 MPH limit without complaint. It made the 6 mile 1400' and 9 mile 1000' climbs with no complaints.
So, for these trips all in all the car performed perfectly acceptably without Mountain mode. With climate or more vehicle load? No, I'd use Mountain Mode for any solid climb over 2000'. Less if steeper.
Interesting observation: A few miles before each of the "reduced power" warnings, the car shifted. I don't know what it shifted from or too, but there was a momentary loss of engine power (and the engine actually spun down a bit) then a re-engage. I'd say it was around 150 milliseconds, give or take 30.
It happened again at 55 MPH on the climb up Laguna, but only about half a mile before making the first (flat) meadow. Likewise it happened at 70 MPH just before making the highest peak at the Golden Acorn on the return trip. In addition, it did it again on a flatish part just after that peak.
Another interesting (well, to me at least) observation: If you've taken a normal depleted SOC car, put it in MM, and floored it off the line, you know what the highest engine rev the car normally will make, right?
After those shifts the engine ran *quite* a bit faster than that! And it stayed there, even for a little bit after normal power was returned. It was WHINING loud!
At four of five of the times the car "shifted" it was under a heavy torque load climbing a steep hill. What I suspect is that as the SOC dropped and the car prepared for reduced power, it declutched the engine (and generator) from the ring gear so that it could run the generator at *above* the max RPM allowed on the ring gear. If so. apparently it can't do that while there's torque going through the clutch, so it neutrals for a moment, declutches, then goes into hyper drive.
Interesting experiment. I don't plan on doing it again.