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?".
Negative effects aren't "expected" - but I've seen multiple instances where early stage companies were paying certain founders too much and getting too little in return. I also noticed that, broadly speaking, founders who were taking less salary were producing more. I'm now wondering if there's a correlation.
As far as measuring productivity - I'm defining it as meeting deadlines.