Well, let me start out by saying that I almost always submit to markets that allow simsubs. Given that, I have never withdrawn a story unless it has been placed elsewhere.
If this e-zine allows simsubs, I'd personally just let it ride out and submit the story elsewhere while I'm waiting.
The bottom line is that most of these places, particularly the web-based markets, are bare bones operations. They get behind. Really, really behind. I take the response timelines in their guidelines to be aspirational. In a perfect world, that's what they're hoping to do.
But in reality, life intervenes. Many of the editors have full-time jobs that are not the magazine. If one person goes on vacation for a week, it can throw the responses off by a month or more as things get backed up. Many markets stop reading the slush as they approach final production on the next issue. Others take significantly longer to respond during the summer for a variety of reasons. Heck, Memorial Day weekend could easily have added a week to waiting times.
Then, of course, there are the positive reasons why a market takes longer. You're getting a second, then third read. You're sitting in a "strong maybe" pile waiting for the end of a reading period. They only have a editorial meeting once a month and your story is on the agenda.
All I'm saying is that I would think long and hard before withdrawing. If you don't want to have a story beholden to a specific market for months, then look at the many, many good markets that allow simsubs. And, IMO, don't write any letter containing an ultimatum until the market has had the story for at least five months. Withdrawing after setting your own timetable could be viewed as annoying, particularly by unpaid, overworked eds who do this for the love of it. Why burn a bridge if you don't have to?