The fact that B keeps the non-MCAS (raw) pitch-up characteristics a secret leads to the conclusion that those characteristics are much worse than just a mild non-linearity. That is probably also the reason why they multiplied the MCAS action by 4 without telling the FAA (and anybody else).
Drastical pitch up cannot be certified (thank God) as it is bl..dy dangerous even when pilots get told/trained. A commercial jet is not a Lake Amphibian and it is operated on instruments , not on seat of pants VFR operation where a pitch deviation up or down is easily identified and corrected.