I thought the slide on the CORE was pre milled.
The thumb safety can make it more difficult to reach the slide go with your strong hand thumb. You're mileage may vary.
I put an extended slide go on my 1911 and never looked back.
The draft rule book is posted on the IDPA site. You must be logged in, I think.
Also, here:
http://members.idpa.com/Content/Rules/mywujz25.qyt.pdf
Edit: Link directly to idpa site.
How do you drop the slide? The thumb safety on an M&P makes it hard for me to reach the slide stop with my strong hand thumb.
It seems like you've made the case for it. You could always remove it.
Edit: it does seem far back and high compared to the 1911 I run. I was able to get on top of it though.