Sim, buddy the DAC you are considering will certainly make a difference to the sound, nothing doubting that. But there are two points that you need to keep that in mind.
(1) Would that be a welcome change, or just for the heck of it. Andrew said it. If you already have an all Arcam set, and loving it to max, I don't think you will be very happy with the change. Why change something you already like. That too at a big cost.
(2) That's where the second point is. Will you be doing justice with that money, investing it into a Benchmark DAC? I agree with Andrew, not a very wise decision. Think of it, you'd be using your Arcam as a transport, won't you?
I saw that you said, you wouldn't like mixing manufacturers for the sake of looks. But still I'd say, if I were you, I'd get a Rega Saturn, Primare CD31 or Naim CD5i.