Modifications to magwell, slide stop, and trigger guard are expressly mentioned as allowed modifications, safeties and beaver tails are not.
To me, cutting barrel length to 4.1" to be CCP legal makes sense, but IDPA has ruled this is not so. If it ain't in the book, it ain't legal, no matter how much we think it should be. I follow the written word, as well as I can understand it, because there is no "spirit of the rule".
As far as ACs go, I listened to one tell a guy he could modify the frame on his Sig P320, but HQ has ruled that illegal as well.
Hopefully 2017 will provide more clarity.