I have done exactly this........the reason why is, DELLS mother board SUCKS! It does not allow the Intel chips to actually post a OC over the 3.9. Even when using the Intel extreme utility, it may say its working......but it does not post to the bios.
The board also has the windows OEM key embedded in the board, thus if you have to reinstall windows good luck finding the CD Key, if for some reason windows cant read the embedded key. This is a problem when you install the MEDIA CENTER in add features to windows.........Dell doesn't want to admit it, but their version of windows 8.1 OEM is NOT compatible with Microsoft media Center edition. This has been documented in Dells Lab and Microsoft has also confirmed it. What happens is, when you buy the Media Center Edition after you install windows 8.1 or 8, Microsoft sends you a CD Key to enter to upgrade to the Media Center edition......well when you reboot after entering that key, windows gets confused and actually deactivates because it is reading the upgrade key as the main key for windows 8. DUH .........Dell is having major issues, and I suspect will be OUT of business in less than 5 years.
The Asus x99 fits in the Area 51 R2 almost PERFECTLY!!! You have to buy a new USB 3.1 header cable and a few USB header cable extensions but other than that........it works great! You can even install the alienware command center, disable the features you don't want and use THE FX lighting to control the exterior lights still. I would HIGHLY recommend this. BUT be warned that if you do.....and Dell finds out, your warranty is over.