Does a drag race only car need exhaust boxes?

My 340 drag only dart has exhaust cut outs with plates that unbolt.
The car was ran on a rolling road for a base line figure and the afr showed a steady 12.
The car then had 2 runs with the cut outs opened. Afr dropped to 11 and then 10. Hp was down 5% and torque down 10% on both runs. Carb is a 750dp. If the carb was rejetted to get the afr back to 12 would the hp and torque be better with the open exhaust or does a car make more power with exhaust boxes over an open exhaust? Or do I need a bigger carb? I suppose what I want to know is what is going to make the most power, an open exhaust or one with boxes or is it not that simple?