It is "mythical" when its peak load is 1/3 that of an OC 480 or 580 while matching its performance.
7850 @ ~1.1ghz is enough to match a 580 generally. I never discount the fact that 480/580can be OC further as well, however the typical air cool results is 10-15% tops. Look at my post above quoting the typo of Ark.
What's amazing is NOT just the OC headroom on it, its the crazy low power use. Even OC it draws 20-30W extra. An OC 480/580 is like the 7970 OC, massive extra power use.
ps. If you look into the huge 7850 beast OC thread, you will see ppl do not claim such nonsense as matching top tier performance (no way 680/7970). Only that it matches 580 perf for cheap ($/w). This was the reason i bought the 7850 and it really did exceed my expectations. The cheap custom 7950 is also the reason why I bought a PCS+ 7950, and I hope i don't receive a dud that can't OC... but thats part of the fun with overclocking, its not a garanteed gain. But even if its a shit card, i'm expecting 1.1ghz from it, at those speeds its essentially a 7970 GE for much less power use.