Ortlieb back-rollers can be modified to go "ultralight".

On the back face of the new (2012?) pannier, there are 9 screws with nuts attaching the top rail, bottom hook, and bottom rail to the bag. Unscrew all of them (regular wrench on the inside, "star" hex wrench on the outside) and lift off the plastic pieces. This allows you to remove the plastic stabilizer sheet and the nylon organizer from the inside of the bag.

Replace all the rails and screws and tension them tightly to retain the watertight seals. The weight of a loaded bag will maintain fabric tension between the top rail and bottom rail, making the plastic sheet unnecessary. The bags are a little unwieldy without the sheet (though never at risk of falling off) when empty, and just fine when full.

On the front of the bag, remove the two screws attaching the plastic hook for the shoulder strap and take off the plastic piece. Seal the holes with some gorilla tape or a sewn-in piece if you're crafty.

Next, throw those shoulder straps in the trash. Attach the buckles from the right bag to the buckles of the left bag instead. Keeps them out of the way.

You just saved a ton of weight! Now your Ortlieb panniers are some of the lightest touring bags you can buy!