Enthusiasm mostly affect defensive ratings.
Wow! I never expected it, but a quick glance through my B3 matches and yes you're right! I played the same lineup for my B3 games, and the defensive rating was pretty constant (since they used the same enthusiasm since game 1) but the huge leap in offensive rating in the second game (before going constant) kinda proves that it mainly affects defense.
I also see an uptick in defense rating as my GS increase too, but much less when compared to the offensive ratings. I guess GS still affects defensive rating a bit, but the impact on offense is more apparent.