I've researched this at length recently. You aren't going to find a 25 degree synthetic bag with those specs. Someone mentioned TNF Propel, and you can also look at the Marmot Pounder, which is rated at 40, but is more like a 50 degree bag. Weighs in at 16-18 oz. A buddy of mine swears by his. He layers and says he sleeps comfortably down to 35 degrees. He basically wears a midweight base layer, synthetic jacket, and hat. I haven't tried his set up, but I am pretty sure I would freeze.
Per a recent thread called "Synthetic Fill," the lightest known 30 degree bags are the Montbell bags, and TNF Fission. In the size you are looking at, they weigh 2lbs 2 oz. Last year's TNF Orion weighs slightly more but is good down to 30 degrees (rated at 20). Other bags are slightly lighter (MH and Sierra Designs) and rated at 32, but unlikely to keep you warm at that temp. There is a Marmot Pounder Plus that weighs in at 2lbs 2 oz. which is rated at 25 degrees, but is actually a little colder than that, according to most.
Other than that, your best and lightest bet is the Cocoon system or maybe a Ray-Way quilt, but if you include clothes (which I don't because I generally take those anyway if it is going to be 25-35 degrees), you can get near you weight.