For outdoor scenes I always use GI, but I tone down the quality to -1.0 or -1.5. This will reduce your rendering time down and you will barely notice the difference on outdoor scenes. The example is a crop of an area from my latest visualization, the bridge face is from the existing photo but the columns, wall, road, barriers and cars are Vue rendered. Notice the subtle shading on the wall as it gets darker the farther under the bridge it gets. This is the affect GI has on a model. I will post an example of a direct comparison on a building shortly.