I've considered adding a "just check the first {user-configged-bytes}" mode, which would offer enough of the speed up you describe, which I think jdupes does (maybe? It's been awhile since I looked at it). I think the speed of opening the file and reading the first buffered page of bytes would dominate the time of the operation, especially if one were to do async reads (which my program does NOT do. I should look into it.)
Worth some performance measurements, anyway!