Actually,i believe Tentastical Insight comes from actual insight given to you by the Great Tentacle Lords, moments before the action is taken. Kind of having a prescience of how events will unfold. Therefore the insight bonuses are actually adequate.

DR shouldn't be a problem when you deal level x D6 damage with a tentacle.
DR20/- is taken care of at level 6 (6x3.5 average damage= 21), and i don't remember a single CR6 monster having DR20/-. By level 20 each tentacle strikes for an average 70 damage (20x3.5) which is quite enough to warrant bypassing any DR whatsoever.