Could a very long password theoretically eliminate the need for a slow hash?
Despite the impracticalities of using a 43-char password, I would say yes, such a long password would be secure when hashed with just SHA256.  Assuming 127 possible ascii characters, a password of 8 characters would require an attacker to search about 2^56 possibilites (viable), whereas a password of 43 characters would require searching about 2^300 possibilities (infeasible, considering that 2^128 for encryption using a 128-bit key is infeasible). Having such a long password would make a slow-hash redundant.  With the case of a 43-character password, I doubt that rainbow tables would be issue either.