[Boofo]

Hook: global_bootstrap_init_start

$maxthreadsgroups = array(5,6,7);  // Usergroups
$maxthreadsids = array();  // User IDs

if (is_member_of($vbulletin->userinfo, $maxthreadsgroups) OR in_array($vbulletin->userinfo['userid'], $maxthreadsids))
{
	$vbulletin->options['maxthreads'] = 50;
}

Change the numbers in red to your liking. You can also use userids. I have settings in a product for mine but this should give you what you need. TESTED and WORKS!