It's hard to say that the bonus, only,can give you an advantage over stronger opponents (or ev3n against any opponent) for me is just a small % but if you use it right and you have good formation and orders you can overcome stronger opponents.
I'm lvl23 and i have played from lvl 1 in +3 cup,won my first cup at lvl13 and in the last 7 seasons I won 6 cups playing against stronger oppo most of times
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