I don't think that Martial Spirit is the single thing that will allow someone to win a war. I mean it's a 25% buff to attack, which comes at a point in the game when your units should already be nicely promoted with 50% or more CS bonus anyway, especially since you picked war trees throughout the game, presumably, which means this is effectively only making them 10 - 15% stronger and only on attack. So I don't see this as being game breaking if it's useful for more than 1 war; it's a nice and useful buff and I think it should simply be adapted to work similarly across game speeds. The balancing issues on Marathon stem from skilled humans still being significantly better than the AI at warfare; you don't fix that by keeping Martial Spirit at 50 turns. If you think that Marathon needs to be better balanced in favor of the AI when it comes to warfare then you should propose changes that help in all situations, not just when the human happens to go Autocracy and picks Martial Spirit.