154 CT is 25.1... 155 CTX is 26.1. 1 cm is a lot when you are talking about width. 1 cm is minute in terms of length.
26.1 is simply much too wide for a size 9 boot. You shouldn't go over 25.5 cm waist.
In fact, with a size 9 boot should not go over 25 cm WW (depending on the sidecut radius, of course - in case of the Proto both the 25.1cm on the 154 and the 24.4cm on the 152 would work for the guy).
Sluggish edge to edge
That. And the fact that it will be much more difficult to pressure the edge effectively, compromising edge hold.