I guess it depends on the size of your show. I have had significant problems trying to do this on shows in the past. It wasn't an issue of 10 vs 100, I know that switches take care of this. It was the fact that the GrandMA only had one ethernet port, and talkign to the NSPs and Artnet at the same time became an issue. Of course, for me sometimes just NSPs even slow down significantly due to the sheer amount of traffic on the line. Your mileage may vary, this is just what I have experienced in the past.
Besides the fact that once your show reaches a certain size, you cannot use the artnet output or built in DMX ports on the GMA, and have to use NSPs.