A gas turbine exhaust diffuser includes a frustoconical portion that defines an interior surface and an axial centerline. In particular embodiments, the interior surface may have a slope greater than 6 degrees, 10 degrees, or 20 degrees with respect to the axial centerline to define an axial cross-sectional area of at least 200 square feet, 240 square feet, or 260 square feet. In other particular embodiments, the interior surface may have an axial length of less than 25 feet or less than 10 feet. A helical turbulator on the interior surface of the frustoconical portion may reduce flow separation between exhaust gases and the interior surface to enhance recovery of potential energy from the exhaust gases.