So the softcaps are basically 14, 27 and 51.

This game would benefit from higher player damage in early game. Spending 3 mins on clobbering down on an enemy while they barely need 2 hits to finish you off is pathetic balance design. Thats not difficulty but artificially enlengthment to tire ppl out.

Attack power is probably what´s killing the game for me. I mistakenly got Shura ending and now I´m on NG+. All the way till Genichiro difficulty seems fine and some bosses (like Lady Butterfly) are even easier with past game upgrades. However, starting on Genichiro and specially now that I´m facing Owl at Ishina´s castle, things are crazy difficult. As I haven´t been further on my first rune, I kind of feel that my posture level is not good enough as I couldn´t upgrade it by reaching final levels and bosses. Moreover, I couldn´t buy the mask, so I´m here stuck getting one shoot kills by Owl unable to parry/deflect most of his attacks. Either I go for a loooong perfect rune just waiting for one attack window (which might happen in years) or I leave the game. What a terrible way of rewarding finishing the game, not having an upgrade system and relying on useless techniques is appalling.

This page seems contradictory with its first example. Correct if I'm wrong, but from what I've gathered here, you should start to suffer from the diminishing returns at AP 15, so the calculation probably looks like this: 30 x (14+3) x 5/4 + 12 x (1) x 5/4 = 657
For the general formula this means that each additional AP has to be weighed by the modifier of its breakpoint. It makes for a real ugly formula, but it shouldn't go unmentioned either in the last paragraph.