Well it's no 911S, but does reasonably well. Going west over the Sierra on I-80 from Reno to Donner summit was a bit of a grind on my recent trip, but the car was fully loaded with a roof pod too. I did not mind hanging out in the middle lane (trucks crawling in the right) and managed to stay at 60 to 65. It probably could've done more, but I didn't want to completely trash the MPG. BTW, I pop my in and out of Sport mode on a regular basis for hill climbing and accelerating onto the freeway. The mileage suffers a bit, but i prefer not to be a complete road boulder. Backing up Traffic is certainly going to depend on where you live as well, meaning overall traffic volume and aggressiveness of local drivers.
Just did a 237 mile round trip going up about 3500-4000 feet at around half way point. Since the climb was a good amount there and back my MPG was 39 for the trip. It was also like 111F in some areas, lowest temp was low 80s in the beginning and 90s on the way back. Had a good 40-60 miles of temps in excess of 100. So the AC was on full blast the whole trip. AC was fine, but I think a Hummer AC I have gets a tad colder. On the way there maintained 70 mph on cruise control fine but it definitely was on power for most of the uphills. The battery got almost completely depleted at the top. On the way back it maintained 65mph well, except in one spot when the battery had just 1 or 2 lines left, with the cruise control on 65, it maintained 60, so I accelerated to 65 and was able to maintain that speed fine.
Conclusion, I had no issue going up the elevation. The 18inch rims actually handled pretty well in the downhill turns. I was surprised at the hit to the MPG, I usually get 47 mpg in every day driving. But I was still very pleased with the mpg performance due to the grade. As a comparison, I'm taking another trip on Sunday about the same miles to a different location, and the times I have done that trip I get about 49mpg going 65mph.