Actually a VERY excellent guess, Darby.
X doesn't have to be THE "head Q guy", maybe just A Q guy that likes Thundercats. Little "inside NSA humor" like Q says.
And I can see them having code names like T, U, V, etc.
Better than Mr Pink, Mr White, Mr Blue. That's been done.
My guess is that "X" is the female member of the team, named after her chromosome.