Have you read this Dell knowledge article?

https://www.dell.com/support/kbdoc/en-us/000178575/alienware-overclocking

How to confirm the over clocking speed?
It is important to understand that the processor will never maintain its top speed all the time on all cores as it will increase the speed depending on the work load.

You can take a single core to max speed to test if the processor reaches its maximum speed by doing the following:

1. Download prime95 by Clicking Here
2. Extract the contents of the compressed file
3. Open prime95
4. Click Just Stress Testing
5. Open Windows Task Manager
   1. Right Click on the Windows Task Bar
   2. Click Task Manager
6. Right click on PRIME95 Application
7. Click Go to details
8.  Right Click prime95.exe
9. Click Set affinity
10. Uncheck
11. Check CPU 0 Only
12. Go back to prime95
13. Select In-place large FFTs
14. Click Ok
15. Go back to the Task Manager
16. Click the Perfomance tab

The CPU should now show its maximum operating frequency

Important: Even after performing the steps above the top processor speed may not be sustained, keep looking at the information and you may see spikes to the top speed

To Stop the Stress Test go to the system tray, right click on the prime95 icon and click Exit

He disabled Speedshift and EIST(SpeedStep) clocks would be locked to whatever the OC was. However on Alienware boards I believe you need one enabled for OCs to work at all