When I look at Chinese vs Japanese calligraphy samples and books, it seems to me that there are subtly different aesthetic principles governing the characters, even discounting variants (including stroke order switches etc) and different formal styles. For argument's sake, let's limit the argument to 楷書 where things would be expected to be the same. Can an expert (or even a knowledgeable layman) see a sample of 楷書 and identify it as Japanese or Chinese? Or are the variations between calligraphers wide enough that national identity does not stand out as a distinguishing factor?
On a per-character basis, generally not. There can be exceptions (e.g. if a Chinese calligrapher is writing 马 instead of 馬), however it's customary in both styles of calligraphy to use the traditional forms of characters.