I agree with you that native resolution is just too darn small. Even with near perfect vision, I just find that it's just a strain to look at all the time and I expect both desktop and Metro to be somewhat usable with touch and that's just not really the case at full res no scaling.
Using the native res, with scaling though will generally result in sharper objects. When you use a non-native resolution, everything will be a little bit less sharp. Because the SP3 has relatively high DPI, it can make things look fairly passable. Still, there is a noticeable difference to me in the sharpness of text, especially, using the native resolution and 150% scaling vs. just scaling at 150% by setting the resolution to 1440x960.
As someone pointed out to you earlier in the thread, you might as well use a fixed resolution to get your scaling factor directly from the resolution vs. setting the resolution halfway there and then using scaling to get the rest of the way. There is always a bit of a performance hit when you have to scale in hardware, so might as well do it by just setting the resolution which has no such hit.