FLG read this debate about tracking versus integration with a good deal of interest. His school district wrestled with the issue back in the 1980s and 1990s. So, he saw a bit of both sides.
He realizes that his personal experience isn't data, but he can say without a doubt that his experience in integrated classes was unequivocally less stimulating and challenging than those separated for ability and it did cause him to become extremely bored and withdraw. So, he is very sympathetic to the argument that focusing on the lowest performing students comes at high-achievers' expense. That doesn't necessarily mean we should shift focus away from the achievement gap, but it does also mean we shouldn't make education policy without remembering TINSTAAFL.