As you can see in the documentation, for example, the Intel Xeon E5 v3 offers a Single-Core Max Turbo Frequency of 3.8 Ghz. Although the hardware boost is enabled, it cannot be guaranteed.
Actually, the clock speeds they quote are base rates, because they can't promise anything higher than that and they want you to get what you agreed to pay for.
But although 2.3 GHz is that base frequency for Intel Xeon E5 v3, as an example, it's possible to get more than that, up to the Max Turbo Frequency (3.8Ghz). To really get the max turbo on a small number of cores, you'd need to ensure that most of the other cores are idle/sleeping, but they don't offer direct control over that currently. For a single core CPU, getting the turbo it depends on the load on the machine.
You might see a fast performance depending on the load on the machine at the time, the more cores that are in use by a program the lower the boost speed. If you're buying X number of core machines and idling X-1 cores, that's probably a good start to get at least some Turbo.