Anybody know someone who will add a few ounces of down to a Western Mountaineering Summerlite Bag? Are there any problems with adding down to a bag? Is this a bad idea?

Have you thought of asking WM to do it?

Hermits Hut can add a free overfill to many WM bags, but they told me the summerlite wont have enough room for overfill without down leakage.

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.