Aumentar limite envio newsletter magento

I wish to increase newsletter sending limit in Magento 1.7 and I've already tried to increase the sending limit based on instructions found on website links down below, that is from default value:

$countOfQueue = 3;
$countOfSubscritions = 20;

to new custom value:

$countOfQueue = 4;
$countOfSubscritions = 1500;

The problem is that magento doesn't sends out more then approx. 1000 - 1100 emails per hour. Currently it should send out 6.000 emails per hour (1500 x 4 cron jobs in magento configuration per 15 minutes). Our regular cron job in cPanel runs every 10 minutes. In WHM/cPanel there isn't any email sending limitation in place, so I really don't have an idea why we can't send out more emails.

http://www.myhostsupport.com/index.php?/Knowledgebase/Article/View/85/33/increasing-magento-newsletter-send-out-limit http://magentoo.blogspot.si/2014/01/magento-newsletter-queue-not-sending.html