I am nowhere near fully leveled, starred and geared, but I went with a DN lead for arena as soon as I unlocked him and have been able to climb up my arena from just under #2000 to #121 yesterday in a month.
I get that Sidious loses some of his special aspects under DN but he has been a solid member of my quasi-Sith team and he works well with his high speed mods to lay down the first debuffs for DN to do his thing, and then also apply healing immunity to key enemy units using his basic during the course of the battle.
I am working on Maul when I can find shards, so he will definitely slot into my team when I have him at the level and gear I need.
For your setup I would go with DN lead, Vader, Palp, SA and ST if you have SA and ST geared well.
I run DN lead, Vader, Sidious, Boba and either SA or Phasma for TM gain for the heavy hitters. I have not found need for an outright tank due to the health steal of the Sith members.
Speeds: Sidious first, DoTs - then Boba to apply AB, then Vader (if there is squishy enemy with couple of debuffs by now I can easily culling blade and go again to lay down more DoTs with Vader). DN is high on tenacity so usually withstands enemy AB on first round, when he goes he can force drain and I end up annhilating one enemy member every battle, as well as fequent dispells using his basic. By the time Phasma runs her Victory March very few of most of my opponents' team has gotten off any special attacks and my guys can go again.