Hi VRBitman ,
Sorry I haven't looked at the delta for 550, I know from a product & features perspective there weren't lot of changes on a relative note of the delta between 5.0 & 5.1(particularly SSO architecture).
Key changes in my opinion would be Configuration Maximum/Minimums were upgraded ,minor Storage and network enhancements.
If I come across any specific guidelines, I will revisit this thread.
-Cedric