It's hard to be certain. My defenders with SAs often get higher ratings and more MoM than the normal players, but I haven't been keeping clean sheets so does it matter? Also midfielders and strikers seem to be getting a high rate of success with free-kicks, corners and one-on-one shots, but sometimes get goals from normal play or fail to score entirely.
I feel better having a few SA players on the field, it might convince the NGE that I deserve to win more often than not, but maybe I'm just dreaming.