Here is the problem i have with this energy approach; in the images above, i rendered a simple square with a single infinite light from the direction of the camera. The left square has a diffuse value of 0.9 and no specular. As can be seen it is evenly lit with a pixel value of 244 which represents gc(0.9). The right square is rendered with almost the same settings, but this time with a specular value of 0.9 and no diffuse. As you can see the right image with the specular value is overall much darker than the diffuse square. In fact, the brightest pixel of the specular square has the same value as any pixel on the diffuse square, gc(0.9). However, the 90% does not mean that 90% percent of the light reaching the square gets reflected, otherwise the specular square should have been much brighter than the diffuse square, because the diffuse square reflects in all directions equally, while the specular square reflects mainly in the direction of the camera. Exactly the opposite is true; the 90% represent peak values, not energy amounts, so when you have a mixed specular/diffuse texture, it does not really make sense to sum those values up and thinking about energy preservation. The only drawback when ignoring those amounts seems to be that you get clipping when you have values higher than 100%, but that is because a jpeg or png file you use to save those images cannot represent pixel values higher than 255. But this is more a problem of the file format and has nothing to do with energy preservation.