While your team is good, and developing your TFP may help your fleet as well, Sion and DN will be your next step forward. At some point your team will reach its limit in the arena. At that point it would be nice to have your Sion and Nihilus ready and geared.
You may have faster characters (mods) than your current opponents in the arena, yes. That's just great. Energy well spent. However, if your TFP always goes first, you may want to move some mods around between your characters to make your Vader as fast as possible. The idea is to have your Vader faster than the enemy Vader, open with Vader's AoE immediately followed by Saber Throw to clear debuffs and hence grant turnmeter to your entire team.