First of all, what are we looking at realistically for Tarkins total Potency? His base right now is around 33% plus lets say around another 40-50% potentially with the right mods? So lets be optimisti...
I believe armor caps at 75%. Otherwise Cody with his +300% defence on a clone team under his own Zeta leadership would be invincible with defence mods.
And I think Tarkim will be a great support character in the HAAT with offense down, but I'm doubtful of his potential in arena. Poor AI control and the prevalence of cleansers being two big concerns.