Not sure I understand? Extended after 10 days..?
Your WP will expire on Aug 31 and you've applied for extension on July 18, right? This means that with the current delays you should have it around October 1.
If you leave the country, they can not treat the application you've submitted online at the border when you come back. You'd have to try to apply directly at the border (with a new form), but like I said, in theory these kinds of permits don't get treated at the POE anymore. If they deny you a new work permit at that point, you may be allowed back in as a tourist, depending on where you're from (if your nationality makes you visa-exempt) and on the officers willingness to help you out. Normally to enter as a tourist you have to convince them that you're intending to leave again --which obviously you don't intend to do... also, you wouldn't be able to work until you get your new permit.
personally I would not take the risk, but I guess it all depends on what you're leaving the country for. Could you extend your trip to come back only once your WP has been renewed?