I think 25% is too OP. 10% can be really useful when your opponent calculated he has just enough to defend and then you play the card. I really like the idea, adds an extra layer to games!
From a balance perspective, let's try to think of 'power' in terms of 'armies'. If you give it a kill rate boost of 10%, then that is often the same as just killing an extra army. Even, if you always round up. I agree this is useful in some cases, but it's not a powerful card. For example the normal +5 armies is much much better unless you have loads of armies in the game.
Now that being said, if someone made such a mod, they would probably make the % tweak-able by the user.