Grand Moff Tarkin Rework - Missed Opportunity?

The way I see Tarkin and Vader's relationship was one of mutual respect. They both understood that the other was exceptionally good at their respective jobs, and so they were perfectly happy to work together. Tarkin doesn't really have any opinions on the Sith as a whole, but Vader he respects.

If they had any sort of synergy, it would be something minor like they get a slight buff to their uniques when on the same team.