No, but I didn't realize that btsync uses 4MB windows. I would have invalided several of those 4MB windows, so btsync would have had to retransfer a large portion of the file. rsync uses 16K windows, so obtains a much larger speedup for small changes, at the cost of being less efficient elsewhere.
Does bittorrent sync efficiently transfer small changes to large files, like rsync? I did a quick test and it doesn't appear to. If it doesn't, is this a planned feature? I can live with inefficiency for a little while, but hopefully soon our dataset will get large enough that it will be a problem.