After realizing how you understood the "121/484MJ each", I think 25% would be wrong even if we agree with the definition of efficiency (measuring point).that sounds right, but than it is unfair to compare this 90% with the steam. The 5% of steam is measured from the reactor's steam outlet (thermal energy) to the shuttle on the launch track. A fair comparison should measure EMLS from the same point, that is certainly not 90%. I don't know if that would be 25% as you calculated, but you are probably close to the truth.
Regardless the actual figure, the overall efficiency of EM launcher is much higher than steam by the same measurement, there is no doubt about it.