They said that 6 should work with new Mac Pros. Nothing about backwards compatible. I specifically asked if my MAC could handle 6gb and they said yes.
So, if it doesn't, how can I make it? If there's a solution?
And the 6 does work with the new Mac Pros, and your new Mac clearly is handling it.
"Will this work?" is rarely actually the question you want to be asking. The drive works just fine - just not at its maximum speed.