I've been looking at some retailers that sell the cobra/heritage/SL and the shortest cobra and heritage I can find in Canada is 155, compared to a 151/153 SL there shouldn't be much difference in terms of float and handling should there? I mean, its only 2cm.
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.