I am not sure extending the QB line by itself is a viable solution. It will just funnel more customers into an already packed line during the rush hour. There is enough delays on the express, especially during the morning, due to the tight headways and adding more passenger is not going to help that. It will simply add to the dwell time as riders try to squeeze on and add to the delays down the line. The majority of riders already jump from local to express now at Roosevelt regardless of how much of a sardine can it can be. Can you imagine if this express/local arrangement was extended further. It will simply add to the delays entering Forest Hills and at whatever the designated express stops are.
This will work if there is a way to convince rider NOT to switch. But GOOD LUCK with that.