Can I really replace my 12k Sidious with 6k Thrawn (arena)

I didn't feel like my SiT was arena ready until 6* (granted I was top 10, not 500). He's a tank, he absolutely needs stars to be effective (though you really do need 5* mods). Also gear, you definitely need the highest gear you can on him to keep him alive.

For the record I ran zzDN, SA, SiT, Dooku, and either Zavage (80%) or EP (20%). Dooku is one of the best pieces in this team, do not replace him with EP. Replace someone else if you want to use EP.