I'll have a thought about the user being able to specify for each patch whether the script should use the desired compost. This is not me definitely saying yes though . If I'd do it, I'd make it dynamic to be able to work with every patch and plant type, I won't hardcode something specifically for a said plant such as watermelons .
I can't give you an answer on 'ban rate'. Bear in mind the XP/H is also strongly dependant on the settings you use, in combination with your level.