Snoopydailydall,
We are Kef dealers with the Kef Reference 3, and the Kef Ref 2C center in a dedicated Theater room
Yes you can use the R series center to very good effect it will blend quite well it just won't sound quite as fantastic as the Ref center channel.
At one time we were using the R center before we could swing moving to the Reference one and the theater sounded good, it was quite audible the improvements going from the R series to the Reference especially in clarity.
The new R series is a bit closer to the sound of the Reference.
If you work with a good dealer they would offer to cell you the R series center and take it back for full credit within a year if you were to upgrade which is what we would do.
Good luck if we can be of further assistance please let us know.
Dave and Troy
Audio Doctor NJ