Dude, you got size 9.5 boots. you are fine! I have size 11.5 boots and ride with waist widths as small as 252mm with a big of heel drag. Center your boots and you are good. 9.5 boots on a 151 reverb rocker will be fine! If you are looking at 390boss's, since you are one of the transition sizes, go with the L/XL because depending on your boots the heel area might be a bit too tight in the S/M. The bindings themselves are totally adjustable so you can customize them exactly to your boots! You will be good to go!
This was already mentioned but, Rome`s customer service is top notch! Any problem with binding parts or your board, you are covered for 2 years! You probably wont need it but if you ever do, they have your back and will get you back up on the mountain A.S.A.P.! I live in western Canada and I filed a warranty claim for a highback on monday, and had a new highback air-mailed to me by friday! No down-time for riding! P.S. 390 boss`s are my favourite binding by far!