Good luck getting a straight answer to this question. Depending on who you ask, a 1" mixed flow drain (that is, one that's sucking air and water) can handle as little as 50 gph or as much as 500-600 gph "quietly." It's hard to put a real number on it because different plumbing arrangements will behave differently, and people have different meanings of the word "quietly." The only way to have a truly silent drain (and a drain that has the least CO2 offgassing) would be a Herbie or Bean style drain, i.e. one that includes a valved full siphon.
I was afraid of that answer haha. Ah well, the only way to know is to test it. I have been slowly coming around to the idea of drilling the tank. checked out a few youtube vids and feel rather good about it (knock on wood lol)
I'm going to shoot for around 10x turnover so around 400gph. For that flow, I plan on drilling 2 1.5 inch holes for the drain and setting it up as a herbie but I'm not sure how big the return pipe should be. I see a lot of people using loc line for their returns and they only go up to 3/4 inch
I also want to know if an internal overflow box is necessary. It doesn't seem to serve a purpose to me. Would someone please educate me on this?