I've raced on DOTs in the rain, here are my thoughts
The most important thing is the quality of the rubber, sometimes rains die before the tread is gone due to long term dry rot. So long as this isn't an issue the rubber should be good.
The tread determines one thing, how deep of a puddle you can maintain traction through. Beyond that it doesn't mean all that much so there's no set amount. I'd replace at 1/3rd wear personally
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