No more than 800 words in trials and I scored a 20 (see sig for which school's trials).
You probably need around 1000 to get some decent character development and plotting done, so around the 1000-1200 point is probably what I would recommend. Rather than aiming to word-spam you would be better off using a little of your 45 minutes to plan, maybe draw/sketch out some of your narrative points, and then get yourself into the feel of your characters and setting before you actually put pen to paper.
Most people treat this section like just another Gatling-gun essay and it's not. You need deliberation and restraint to score well here. Don't try for anything overly complex; instead, focus on evoking a distinctive mood and tone with as few words as possible. Hemingway would've killed Ext 1.
Don't focus on word count - focus on making your words count.