I've been running my 7870 OC'd for more than a year now, and it is showing no signs of reliability issues. I very much doubt that the reduction in lifespan of the card will make much difference, as it's likely to be replaced long before it fails. (It'll likely be replaced with a 300 series when they come out)
You might decide to buy an R9 270, and save a few bucks over a 270X. As for a factory overclock, they are generally done at stock voltage(what I have done personally), and will have minimal impact on the life of the card. The increase in heat and power consumption due to the OC will also have little impact on the life of the card.
You can probably reasonably expect the cards to last 2-3 years of HARD usage(mining, etc). Since yours will likely not be pushed that hard, it's probable that the (useful) lifespan of the entire computer will be more of an issue than the life of the cards (or any other component, really).
One other thing to consider, I know you said you'd prefer to avoid Sapphire, but, unlike Asus, MSI, Gigabyte, they build there coolers specifically for AMD cards. And they are usually cooler/quieter. Asus, has a tendency to use the same cooler for both Nvidia and AMD, and it's not as well optimized as a dedicated cooler. That said....we are talking maybe 5 degrees and 1-2dB difference, max.
White Box: NZXT H440 White, i7-4790K, NH-D14, MX100 512, GSkill Trident 8x2 2400, Corsair CS750M, Sabertooth Z97 Mk2, Sapphire 7870OC Dual X
Black Box: PS07B, X560, F1A75-M Pro, A8-3870K@3.5, 4x4Gb Ripjaws X 1866, Intel 520 180GB, Asus DRW-24B1ST, Sapphire Ultimate 6670, Mugen 3
Square Box: HP Microserver Gen7 N54L, 2GB, 250GB Seagate HDD, 2x2TB Hitachi HDD