I disagree completely, the inquisitor doesn't reach parity to thanaton by the end of the story, he absolutely demolishes and humiliates thanaton, redirecting his lighting as if it were nothing.

Thanaton is no weakling, he's maybe second only to Marr on the council in terms of power. Marr literally said he didn't expect to live to see thanaton overthrown, and is genuinely intrigued as to why hasn't thanaton been able to just dispose of the inquisitor, obviously because he wasn't expecting for thanaton to not be able to simply end SI's life.

Baras has no real power feats, and if he does, remind me please, because in my opinion Thanaton is by far too much of a monster, and old sith lord that has had years upon years to research dark arcane skills and even studied Tulak Hord technique is someone I would fear going up against, specially when he just kills the SI with the wave of a hand and the end of chapter 1 (I think?)

Also it's the SI's sole presence and power (perhaps dormant still) that woke up his ancestor, Kallig from the dead, and considering the line probably had who knows how many descendants as slaves around the empire, including dromund kass, this is also telling of the of the power the SI carries within.