They will probably be big for the S/M and small for the L/XL. I went from 8.5 Burton Rulers to 10.0 DC Scouts and I have the S/M bosses. They are pretty much maxed out and fit all right. I'd say go with the S/M.
Burton Ions have shrinkage so your size 8.5 has the girth of a 7.5 boot. Length of course is still the same as no amount of tech is going to change your actual foot size
So all signs point to S/M sir. I can't stress this enough, make sure you read the instruction manual thoroughly. There are lots of adjustments that you can do to them.
I convinced Snowjab to fiddle with his Targas more before selling them and he did. Now he loves them. I swear, if everyone having sizing problems with the Bosses lived around me, I'd visit each one and set them up myself
If I can fit my very low profile 9.5 Hails in a L/XL, I'll be darned if I can't fit size 10.5 DC boots in them.
So the moral of my story is, I recommend the S/M. Have fun adjusting that ankle strap first time around. Locking the inner tab into place is a mofo the first time around. I highly suggest you wear some leather gloves or something to help relieve your thumb of some of the abuse haha.