I am in bracket 1 and the rewards are fine at that level.
I agree in non-competetive brackets the winner should get 50 percent more. By non-competitve I mean guilds where going up or down a bracket has no true impact on your likelihood of winning.
The real issue is in the top 4 to 5 brackets the standard of play varies by bracket. If you offered bonuses for winning you might get perverse behaviour where guilds tank to drop down brackets so they can win and get the bonus.
Possibly the devs can put in a finish first bonus for all guilds outside the top 5 or 10 brackets. Then top brackets where standard of play varies have no incentive to lose, however winning will get them moving up brackets and so higher guaranteed rewards. Lower down brackets then also get the incentive to play hard.