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Yur, you seem to be forgetting a crucial point: with the current upgrade priorities, Swamp creatures generally level up better than Barbarians, they have a higher chance of getting the desired skills, and weakening their special ability won't fix that.
However, the overall balance of Kreg-Taz isn't particularly important; what's more interesting is the relationship between Taz, Brogh, Vistan, Dragon, and Alkin. My opinion is that Taz with 5% will be much stronger than the other Swamp creatures.