I'm using 3/8 supply, 5/16" return pre-bent stainless steel lines from Fine Lines. Having to slightly tweek the bends is not a problem, heck they come bent in the box and you have the straighten them anyway.
I think if your using a 350 lph or smaller pump, the smaller return line should not be an issue. I think the return port in the built-in regulator is less than 1/4"?