You will lose the daytime sky by forcing shader model 3 if you do so without enabling HDR lighting. For whatever reason, you need HDR lighting for shader model 3 to work properly. I personally used this tweak with a budget HD5570 card and was surprised to gain a significant FPS boost even with HDR lighting enabled. I have no idea how modern graphics cards are made, but for whatever reason my HD5570 works better with shader model 3 than 2.