Just based off of some quick measurements and calculations on my 156, it LOOKS like the 165x will be around 28 cm at the inserts. This of course presumes that the waist and insert width proportions are similar. I could be way off though so best bet is the physical foot method. You could also try to find a shop that has one in stock and ask them to measure at the inserts.
Going home and putting my bare feet on the board made my decision even harder. I thought I made up my mind to stick with the regular 164 but when I put my feet on the board at the inserts, my feet hang over on both sides by about 1/2". Bumping up to the 165 wide model would add about .6" to the width and make my feet hang over on both sides of the board by about .2". That sounds like the perfect width, does it not? Backcountry.com will take my 164 board back no problem but if I switch my mind up again after I get the 165 wide I don't want it to seem like I am taking advantage of them by doing yet another return. I don't want to be "that guy".
Maybe I will get more days in on the 164 in and test the actual drag out in pow conditions. Then if I want to go wide I can just grab the 2014 Raptor X... call me OCD but I want this to be my "one board quiver" board for a long while. Another reason this is hard is because my Burton Triumph was a mid-wide and it's waist width was right in between the 164 Raptor and 165 Raptor X.