You'll get more flow out of it if you run a larger return pipe. And yes, typically a 1" drain should keep up with a mag9 unless there is virtually no head loss. (usually a duroso/stockman/other vented drain can handle @600 gph - OR - if it's a herbie, or Bean siphon drain then 1" is usually good for around 1500gph).
You should be able to look up the flow chart for the mag9, and go from there. Keep in mind that Danner creates their flow charts assuming a larger return (1.5" on the larger pumps, I'm not sure what they use for the mag9, but I'll bet it's still bigger than the fittings on the pump)