Scientists affiliated with the RIPE (Realizing Increased Photosynthetic Efficiency) Project at the University of Illinois and the U.S. Department of Agriculture Agricultural Research Service report that they have been able to increase photosynthetic efficiency in genetically engineered tobacco plants by 40% over normal tobacco plants.
They did this by working around a well known problem in many types of plants. Instead of only taking in CO2, the main enzyme involved, rubisco, also can bind oxygen. This not only doesn't produce the usual carbohydrate that is the base of the food chain, it creates toxic side products that the plants have to spend energy to break down into safe forms.
The key thing they show is that they can do this not in the laboratory, but in ordinary fields here in Central Illinois. Tobacco is a common "lab rat" plant, so it's not about the tobacco industry. Many of our biggest crops (so called C3 plants) waste energy this way. If they can do it for tobacco, they probably can do this for other plants as well.
Original Science Paper (may be paywalled): http://science.sciencemag.org/content/363/6422/eaat9077 [sciencemag.org]