My Windows 8 Pro with Media Center key is really well decoded by DigitalProductId using the StartIndex = 52 and I've no problem with it. Take a look on your code again. And see if you're decoding the DigitalProductId from 'SOFTWARE\Microsoft\Windows NT\CurrentVersion'.
I've messed up this important part of the thread. The Pro key is really rewritten with the Upgrade key.
Yup thats right, and sometimes the old product key can be found in. HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\DefaultProductKey(2)
The decryption methods of RC and RTM are the same. This Upgrade key replaces your current activation completely. Upgrade keys is Retail keys that needs of the Windows activated before convert the Pro to Pro WMC.