I reliably do upwards of 1.8 mil damage in the first two phases of t4 with DN Lead (g11), Palp (g11), Sass 5* (g10), Sion zeta 4* (g11) and DV (g11)
I don't know how much will change as phases get harder, but keep in mind there is plenty of room to grow on that team. It survives fine without SiT feeding tm to the boss every counter. Vader could probably be dropped, and perhaps Palp (he does bring offense up and has two specials to avoid defense down against DN)
Sidious inflicts debuffs more reliably than most because he applies dots before he does damage unlike most of the roster. That said, Sion getting hit with an aoe is an equally reliable method that will only expire due to the poorest of rng.
That said, most of us won't have Sion starred for months.