dynamically create callback functions inside a for loop

Probably you can do it with args:

'callback' => benz_new_product_tab_content,
'args' => '_tabs_content_' . $y

And your function:

function benz_new_product_tab_content($param, $args) { 
  global $post; 
  $table = end($args); 
  $benz_tab_content = get_post_meta( $post->ID, $table, true ); 

  if (strlen($benz_tab_content) > 0) { 
    echo $benz_tab_content; 
  } 
}

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One alternative is to create anonymous function like this:

  'callback' => function() { // do your work }

Typically to create functions in FOR loop you would use eval. I think this is not the smart choice.