1) Belichick decided that Arrington wasn't worth $3M as a slot receiver. Apparently they couldn't work out a new deal (we usually call those restructures or extensions). Arrington could look around the league, find out what he's worth and come back. It does seem unlikely.
2) McClain is a decent slot corner. I didn't understand why he was signed. He's pretty bad on the outside. Backup slot corner isn't usually a roster spot. Now I know.
3) This cut doesn't affect the outside corner situation. We still have Fletcher and Butler, with Ryan as the backup, same as yesterday.
4) So, we have four corners pretty well set, before the competition begins. We could trade for someone. A vet would be cut who Belichick values more than others. McCourty could be moved to corner. Chekwa, Green, or Roberts could make the 53.
5) The savings is $3M pure and simple, Arrington's 2015 salary. There is no other money that is relevant. It really don't matter that half can be moved to 2016. There are lots of ways to add cap space to 2015 if such space were needed.