We had two marine's attend and pass A&S about 2 years ago, but they couldn't get orders for various administrative reasons. They did better at A&S than most of the active duty guys, but they are also both studs. Overall, reservists tend to do a little better physically than active duty guys because we're a little older, and we make our own PT schedules. The Crossfit guys usually dominate. If you're not training Crossfit, I highly recommend you start, and do all your WOD's in boots and body armor, and all runs with a ruck on. It's good practice. If you can handle that and you're a decent swimmer you'll be fine.
With the updated MARADMIN out (
http://www.marines.mil/news/messages/Pages/MARADMIN410-10.aspx), getting selected shouldn't be a problem as long as you can handle the course, and your package is in order. The Commandant is adamant about getting MARSOC up to strength, so I wouldn't expect a lot of resistance to getting the job as long as you're capable.