IMHO there's a 'Goldilocks' zone for their pay. You want them earning enough so that they don't need to take on a part-time job, but not so much that they're content with the status quo. Keeping the founders "hungry" (figuratively) is difficult, but anecdotally I'd say it's key to fostering a winning culture.
The question is badly stated. Asking about "paying founders too much" suggests that negative effects are expected. What you seem to be asking is "What is the relationship between founders pay and their productivity?". That should lead to another question: "How do you measure productivity?".