There's no need to over engineer. A single 3/4" can easily handle over 1500GPH flow rate, which is more than enough to give this tank a min of 7X turn-over rate. Keeping the hole smaller also increase the structural integrity of area inside the over flow foot print.
+1, this is also answer my question, because my new 150 gal tank order has 2ea of 1.5" for drain and 2ea of 1/2" for return. I plan to have 3/4" start from pump then reduce it to two each of 1/2" when it go to overflow before water go back to the DT.