That is correct, in my opinion. 2cm is not that big of a deal in overall length. One thing to look at is effective edge on the NS boards. For example, I upsized from an old SL 155 to a Proto 157. One thing I did not account for is the newer NS boards that have the blunted tip/tail allow for a longer effective edge than the overall board size would normally indicate. Simply put, they ride bigger than their size. Although I only went up 2cm, it "feels" like a bigger change. I do not think this is really anything to get hung-up on, just make sure you are looking at apples to apples when evaluating sizes.
Seeing as how you've ridden an SL before, how would you rate it in terms of a freeride board? Given my size I think a 153 should fare pretty well as I haven't really gotten into back-country snowboarding yet and shouldn't be in and more than a 2 feet or so of powder