^ Actually, that's not accurate. 7870/270X overclocking is not going to be comparable to 270 overclocking due to the missing PCIe power connector on the 270.
There's one review that demonstrates overclocking, and it's not pretty:
http://www.hardwarecanucks.com/foru...ews/64083-amd-radeon-r9-270-2gb-review-8.html
This shouldn't be surprising at all - this card is seriously power-limited. AMD needed it to slip in under the 150W limit, but that means it's pretty much already there at its stock clocks.
Short answer: don't buy it thinking you're going to get big overclocks out of it.
It's not the single 6pin. 7850's had no issues O/C'ing and they have a single 6pin as well. Stock voltage is likely what's holding it back (could also be simply silicon lottery). With some extra voltage and a better cooler, it should reach the same clocks as any other Pitcairn.
The 7850 used way less power at stock speed than a 270. And by the way, all three 7850/7870 cards I've tested were voltage locked, so extra voltage is definitely not always an option.
As usual, MSI includes their AfterBurner software, which is one of my favorite utilities for GPU overclocking. Unfortunately, I wasnt able to get any voltage controls, but increasing the power target to +20, I was able to hit a solid 1150MHz core (20% improvement) and 1500MHz memory (7% improvement). Remember that the R9 270 Gamings core clock is already overclocked beyond AMDs reference design by 50MHz (~5%), so the R9 270 Gaming definitely has a ton of overclocking potential.
Like with the AMD HD 7850 the overclocking on the R9 270 is heavily limited by CCC restrictions. In the case of the R9 270 to no more than 1050MHz core and 1500MHz memory. However, MSIs Afterburner (which I use for overclocking) along with most other GPU overclocking utilities (like ASUS GPU Tweak) allow you to circumvent these restrictions easily on the R9 270. By doing so we were able to boost that overclock from 1050MHz/1500MHz to 1154MHz/1520MHz up from the stock factory speed of the graphics card which is 975MHz/1400MHz. To add another set of confusing numbers to the mix remember that is already up from 925MHz/1400MHz reference speeds of the R9 270 GPU.