There's plenty of room for more down in a Summerlite. However, it's not a bag that WM has an overfill for... and they seem to be quite hesitant to do.
Not a problem adding the down, shoot, relatively easy to do on your own. Just un-stitch a zipper, add your down, and sew it back up. If the thought makes you uncomfortable, there might be some people on the site willing to undertake the (paid) project for you. I don't know if Tim Marshall of enlightenedequipment.com would do something like this or not; I think he's been busy just making his quilts, but maybe? Best thing to do is probably just make a forum post requesting someome to do the modification for you.