Tar: kill the error message: ‘tar: Removing leading `/’ from member names’

First: Backing up tar files instead of single files is highly recommended to avoid the shoe shining effect, which is what you experience: The computer can't deliver files fast enough and the tape drive has to stop and before starting to write again wind back a little to find the precise point where the stream ended. This isn't only much slower but puts a lot of wear on both the drive and the tape (modern drives, i.e. LTO4, are said to be better at preventing/reducing this effect as they slow down when their input buffer runs empty and don't need to rewind).

Second: It is possible to skip damaged sections of tar files, at the very least for uncompressed archives.

Third: Bacula indeed can (and should) be configured to create a spool file which is then written to the tape. Unfortunately, it is unable to spool to a spool file and write out another to tape at the same time, effectively reducing the backup speed by ~50%.

The first way you invoke the command doesn't create a pty, while the second may or may not. Try telling sshd not to create a pty. In your authorized_keys file:
command="/usr/local/bin/myscript.sh",from="client",no-pty ssh-rsa...

If you want to ssh from the command line when the no-pty option is set in your authorized_keys files you must redirect the STDIN to read from /dev/null or you would get an error saying PTY allocation request failed on channel 0:

$ ssh auto@remoteserver < /dev/null > content.tar.gz

If instead you do the ssh command without a controlling terminal, there's not STDIN available and you can omit the < /dev/null part. So in cron you might do the following without problem:

0 4 * * * ssh auto@remoteserver > content.tar.gz