TC + Psychokinesis isn't an optimal pairing, largely because TC has no real AoE (LoC invocations stand as the giant exception to that rule of thumb) and Psychokinetics has very little AoE as well, since the bounce from Electrokinesis is pretty minor. It's nice to add in Metathermics to either of those two schools because meta has huge AoE.
That said, 70 Metathermics is certainly workable. It's a solid support school if you want to use it that way - for example, as you point out, capping ThermoD damage and using your high single-target damage from TC or PK to kill the walking bomb. You're into mid-game if you're at Core City, so pretty soon you'll start finding psi cost reduction, which can offset the penalty of using 3 or 4 schools. You can afford to dabble in all four if you want - really only need to innervate 1-2 TC abilities, 2-3 each PK and MT, and just 1 from TM is fine. You can fit all that in at the same time.
You don't need to reroll if you don't want. Grab the psi feat that reduces cost penalty and you can run quad-psi just fine once you've got a muffled psi headband and a psi beetle tac vest.
