Benches are in.
First off, I seem to have acquired a mediocre card for OC'ing. The particular card I chose, the GV-R785OC-2GD, comes overclocked from the manufacturer. (975 Mhz Core vs 860 Mhz). With software utilities, it will go up to 1175 Mhz Core and complete a 3DMark'11 run, but it will crash within 10 minutes of playing Crysis or running Metro 2033 benchmarks. Max stable core for prolonged use is no higher than 1150 Mhz.
In order to get clocks that high, I have to use Sapphire's Trixx tool. I'm sure you all know, Catalyst only allows core speeds up to 1050 Mhz. In addition to Core/Memory clock speed changes higher than Catalyst, Trixx also includes a voltage slider that goes up to 1.225 volts. I really wish the Gigabyte had more voltage headroom as I'm certain it has more potential than 1150 Mhz on the core. Considering the core is nearly an identical chip to the 7870, I'm sure it would work fine with 1.3 volts and open the card up a lot more.
The memory chips are another story. The RAM used on this card are the Hynix H5GQ2H24MFA-T2C, or the same ones used in the 6950s; they are rated for 1250 Mhz (5000 quad pumped) and have a lot of headroom beyond even that so I'm a bit perplexed as to why Gigabyte didn't clock the memory at full speed if they went out of their way to overclock the core. RAM is rather, detuned to 1200 Mhz (4800 QDR) but I found I can go up to 6000 Mhz before I see artifacts. General consensus is that anything past 5500 Mhz is useless and actually detrimental to performance on the 7850; I will try to see if there is any merit to that.
For testing, I wanted to use games that actually push the card and not hover in the 100+ fps, because, at that point, who needs to overclock anything anyway? The obvious choices were Crysis (still after all these years, I've never played this game at Very High settings smoothly), Metro 2033 (an extremely demanding benchmark, very cool lighting work), and to round the whole thing out, 3DMark 2011.
The settings I used were:
1. 3DMark 11 - Performance Setting
2. Crysis - Low/Medium/High/Very High DX10 - 1920x1080 : 2x AA
3. Metro 2033 - DX10 - High/Very High | DX11 - High/Very High - 1920x1080 - AAA
On to the results -
'Gains' in this table are calculated using only the 'Graphics' column.
3DMark suggests Gigabyte picked a good stock OC, which gives a ~5000 point increase from reference clocks, then a bump up to 5500 on the memory gives a respectable 27% performance gain over stock. Each interval of clock increases indicate a 7-10% step in increase until RAM is maxed out at 6000 Mhz; 3DMark (somewhat expectedly) suggests that an increase in RAM speed past 5500 Mhz doesn't net any extra serious performance. A 10% increase in speed from 5500 to 6000 only nets 1% more performance. Probably not worth the extra heat and stress, especially because the RAM chips are cooled naked by air.
Optimal Clocks: 1150 Core/5500 Memory
'Gains' in this table are calculated using only the 'Very High' column.
Crysis results mirror those of 3DMark. At 'Very High' settings, we see a nice jump of 10% over stock clocks at Catalyst max settings. Everything past that are miniscule, a 33% Core/25% Memory increase nets only ~12% of extra performance. I've probably got a bottleneck somewhere in my system, probably CPU limited.
Optimal Clocks for play: 1150 Core/5500 Memory
'Gains' in this table are calculated using only the 'DX11 V. High' column.
Metro 2033 is the only test in which RAM speeds actually made a performance increase all the way up to 6000 Mhz. Metro 2033 is also the only benchmark that made my card as hot as 67 Degrees Celsius; this is one demanding game. Too bad I'd never play this game at Very High settings anyway as it is still a lag-fest, so at DX11-High settings, there are no performance gains to be had past Catalyst Max OCs.
Optimal Clocks for play: 1050 Core/5500 Memory
Conclusion:
The Gigabyte version of the 7850 seems to be a great card if you want a cool and quiet (not once did I hear the fans on this card spin up) solution to mid range graphics, but you'd be better off getting a different brand with unlocked voltages if you plan on overclocking. It's really too bad as the cooler seems more efficient than other brands (I know of MSIs and XFXs getting to 80 degrees and higher) and the clock speeds scales incredibly well in measurable performance.
Overall I'm pretty happy with the card regardless of falling short of the fabled 1200 Mhz core figures as in any of the games that matter, it only nets maybe 3 or 4 frames extra. I really wanted Battlefield 3 to run constant 60 fps at 1080p at High settings and this card does exactly that at Gigabyte's own OC level.
I'm going to leave the card at 1050/5500 in Catalyst and not use Sapphire Trixx as that provides a decent performance boost and I typically don't hit the bug where reverting to 2D clocks doesn't happen properly and I get a vertical line screen crash. If anyone has any idea how to circumvent that, I'd love to hear it. Example: Going from playing Metro 2033 to playing a flash video on youtube in Firefox causes my desktop to crash, every, single, time. Quirks.