Re: how can I reduce the overscan
There's an "Overscan Compensation" setting in nvidia-settings; I found it under GPU 0, CRT-0. Increasing that value causes the GPU to scale down the image to fit in a smaller region of the signal; since this leads to resolution loss, is also enables a sharpness filter when active. Value 0 there means using the whole mode, i.e. the largest possible image, and may be overscan for your display. I hope this helps.