DATETIME2 and IDENTITY as composite key
Don't confuse the primary key with the key of the clustered index. Though they are often the same, they are independent.  You just need your primary key to uniquely identify a record in your table across all the databases that have a copy of it.


  Since recalling this data needs to be performance driven, I know right off the bat I want a clustered index on DATETIME2.


I agree with this choice, since you will primarily be writing, reading, and merging data by this column.  Be sure to pick the right precision for your DATETIME2.

As for the primary key, I suggest making it (system, id), where system is a unique system identifier and id is an IDENTITY value. A CHAR(1) or TINYINT will suffice for the system identifier if you have only a handful of systems. 

Don't bother managing the ranges of the IDENTITY values across the different systems.  They can overlap as they please since the system identifier will distinguish records from different systems that share the same id.