When mcrypt is working in openpgp mode it encrypts whole input in memory: output file is written from buffer after encyption is done (rfc2440.c/pgp_encrypt_file).
=> unusable for very large files.
Also --flush is meaningless in openpgp mode.
From man page it is not obvious that mcrypt works in --openpgp mode by default. I would also suggest mention --no-openpgp explicitly.
Thanks
Jan