I agree with Challenge 100%, as I just built a 3930k system that had a 690 in it for a few days, then I went to 4gb 680 in SLI. What he describes is exactly my experience.
Now, that system is on 3 screens, so it's hard for me to judge the question the OP asks, but I just had a new box built for my bro in law that is staying with us for a little bit. Specs and results as follows:
X79 Sabertooth 2011 MB
i7 3820 CPU@4.6ghz, h100i cooler
16 gb Corsair Vengeance Ram
128gb SSD drive w/1tb data drive
680 GTX FTW+ eVGA GPU
Win7 64bit
Asus 24" 1080P LED
With this system, with the entry screen video setting set at 2048 texture, one notch from the top anti aliasing, and everything MAXED in the control panel in game, the avg FPS is under 50 I would say. Taking off from a base with some friendlies and smoke is around 47-52. Flying low over tanks at your town with some cons, as low as 35, but around mid 40's avg. Up high, so long as no clouds are around, you CAN see 59/60, but this isn't the performance I see often.
Now before you say it's the settings or something not set up right, this PC has the full black viper treatment, and I tried it in AH both before and after this, and with the current nvidia drivers AND the ones most people say are the best, from a few months ago. Same result in Aces High. The system also has been through the 3d Mark vantage and dx11 tests, and scores right around where similar systems do on that site, even slightly higher when I've overclocked it to 4.6ghz. Guys in the game that have these 3 and 4 year old systems that claim to have "60 constant" at full settings are dreaming I think. I can post lots of screen shots to verify this. In order for me to get 60 fps semi constant with his system I have to turn down the shadows to 4096 from 8192 or whatever, turn off the cloud bump mapping, and move the updates slider down several notches to where it is only a couple from being completely off. Even then i see drops to 50fps at times.
I'd love to see screen shots of guys with these older systems with cons and smoke and clouds around, at 1920 res with max settings, with the needle pegged at 60 fps, as I can't see it happening without a LOT of stuff turned down or off.
I'll be happy to send anyone dxdiag and screen shots if they want to see them, but this is how AH performs on this system with everything TRULY maxed and at 1920 resolution with vsync on, and the other settings as I mentioned. It is playable at absolute max, but when looking around when at low altitude with lots going on, you do see a noticeable "chugging" going on when it drops into the 30's. As I said though, this system really performs well in the 3dmark tests, and in games like BF3 and the like, they can be played at ultra and see 55-60fps most of the time. AH can really challenge a system, even a new one, with everything absolutely maxed. My system on 3 monitors is a bit more powerful than this 3820 1 GPU machine, but not my leaps and bounds. I may try it on one screen with the same settings and see how much of a difference there is in performance from a 3930k sli box to a 3820 single 680 box.
edit: I just ran 3dmark11 for kicks, here is the result.
ScoreP10154 with NVIDIA GeForce GTX 680(1x) and Intel Core i7-3820 Processor
Graphics Score
10243
Physics Score
11196
Combined Score
8433
Like I said, very common performance numbers when I compare it to other testers on the 3dmark site, the scores are actually slightly better than the average 3820 with a 680 from what I've seen there. I've tried messing around with the settings for the video card in nvidia's control panel, trying Aces high with various things both on and off, and with the card overclocked with MSI afterburner utility as well, and still, there are drops into the mid to high 30's when around gv's at a town, and some other activity in the air happening. It's kind of disappointing when a 2200$ system can't max out a game that is 10 years old almost, lol. Oh well, he'll have to be happy at 4092 shadows and 2 notches of environment. One thing that is odd I find is that we can go offline with these settings and the fps do stay pegged at 60fps, flying through the ack and low over the town etc. I wonder why it drops so much in game...is it just having other players around, and the smoke from burning hangars and the like when at low altitude? I'd love nothing more than somebody to come on here and say "hey moron, you have x setting wrong, fix this and poof, you'll have the constant 60 fps in game that all these guys with 3 year old systems are claiming".