You know a lot better than to trust a generic snowboard length calculator
But generalizations are what I do best!
I have to stick to what I know!!! lol
Ultimately I don't think it'll make a big difference either way between the 155 and 158. Some people may get wigged out by a larger number and "feel" like the board is vastly different than what it is.
I know the opposite happened for me recently. I bought a board with nearly identical specs to my old board in terms of overall length, waist width, and tip and tail width, amount of camber, etc. Therefore I expected it to "feel" like my old board under my feet. What I was ignoring was a much larger sidecut radius and effective edge, which made me feel like I had been riding a harley and now I was on a sportbike. Felt completely different.
In contrast, when I went from a 153 freestyle board, to a stiffer, longer 159 board it should have felt very different, but it didn't. Because the sidecut radius and effective edge were similar.
Next time Burton has a demo day at one of my local mountains I'm going to see if I can get the Custom or Custom X in two sizes, and go out and try them back to back. I honestly don't think there will be much of a difference between two close sizes.
Happy riding all!