Look around the web, there are multiple factory-OCed 280 reviews out there. I would say that a factory OCed 280 is only worth it if:
a) it's pretty much the same price (or not much more expensive) as stock 280 (examples: MSI OC or ASUS TOP) or
b) it's watercooled and you're building a silent PC (EVGA HC or BFG H2OC).