Realistically, I believe BYU would have $10M of tuition going to support athletics, regardless of whether tithing was involved or not.

So the involvement of tithing to subsidize tuition isn't changing how much money the athletic department gets, it's just changing how much what the students pay. Keep in mind, with 36000 students, 10.3M is only $285 per student, much much less than any tuition subsidization accomplished through tithing