I don't think it's the exposure you want to adjust chornor. If you look at the edited image, the clouds on the left are becoming over exposed to the point of cloud details getting lost. Also, by increasing exposure you also increase noise making the image grainy. In addition, when you increase exposure, you increase the light across the entire image where it would be much better to increase the light in selected areas of the image by capitalizing on shadowing, which you can do with a light source such as the sun. And lastly, too high of an exposure and your colors start to become over saturated making the colors start to look garish and blowing out valuable mid tones. Keep the expsore at normal (Terragen's "normal" anyway) and introduce light into the image using a light source such as the sun. Control the light. Your image is well worth fussing over because you've captured depth using a good balance of foreground, mid ground and far background. Don't let even the slightest blemish persist in this image.
-Pat