In trying to answer the question of "what data if any backs up the 1500 hour number as part of the requirements for obtaining an ATP certificate", and hence being qualified to fly for the airlines, you would need to dig into FAA rule making history for insight into that. That number did not come from Colgan 3407-driven legislation, it was simply tied to FAA rules for getting an ATP which were already in place. Be that as it may, the argument was presented by the Colgan 3407 families that if CA Renslow had not been hired with such low time (~650 hours) he would have been better able to prevent a deep stall. This is an argument that bears apparent logic although I doubt if there is much empirical data backing it up. FAA has identified dangerous experience (flight hour) levels, that data is available, but I do not think it contains such specific information as to the exact number of hours a pilot ceases to substantially exhibit dangerous actions like those which downed 3407.