Using a Noctua NH-D15S equipped with a single fan and overclocking by using the BIOS, we managed an all-core, stable speed of 4.1GHz. Voltage was increased only slightly, from 1.3625V to 1.375V. The speed uplift isn't that much, as the chip runs at 3.7GHz under standard all-core load.
There's good reason not to be too miffed by what appears to be limited headroom. A new architecture and manufacturing process rarely offer huge headroom for big chips. Top speeds should hopefully improve as the manufacturing process is refined.
Running a trio of benchmarks shows that gains over the stock configuration are, as expected, limited. Pushing our review chip to the brink, we also set our Noctua cooler to high speed, raised voltage to 1.45V and attempted to run at 4.3GHz on a single core with all other cores disabled. The 1800X managed to complete one run of PiFast, but returned a score of only 18.7, which is still some way off the Core i3.