This isn’t what get_template_part was for, get_template_part essentially behaves like PHP’s require function. Justin Tadlock writes a lot more about this here and also talks about a WordPress function that might be more useful to you – locate_template.
Alternatively, if you did want to hack this functionality using get_template_part, you could use template buffering:
function load_template_part($template_name, $part_name=null) {
ob_start();
get_template_part($template_name, $part_name);
$var = ob_get_contents();
ob_end_clean();
return $var;
}