If you start playing with the custom modelines, you should be able to manipulate the overscan and centring. The objective being to find a set of timings that the screen will accept, and that remove your overscan.
ModeLine "1920x1080" 148.5 1920 1960 2016 2200 1080 1082 1088 1125
The first number is the pixel clock in MHz. So here, we have 148,500,000 pixels per second.
The rest of the numbers all indicate timing positions as the image is scanned out onto the screen from left to right, top to bottom, in pixels (not seconds!)
The bold numbers are the actual number of visible pixels.
Then the italic numbers are the start of the sync pulse that tells the beam to return to the left or top of screen... so here, the horizontal sync starts 40 pixels after the right edge of the screen.
The underlined numbers are the end of that sync pulse. So the horizontal sync pulse is 56 pixels wide.
Finally, the unhighlighted number is the position a the end of that scan line or frame.
You need to keep the 1920 and 1080 the same, of course. And note that the 148.5MHz pixel clock is the entire frame multiplied by the refresh rate (2200x1125 pixels x 60 frame per second = 148.5Mp/s or MHz) Ensure you recalculate this if you change the overall size of the frame, and that you don't go too high with that number as the screen will eventually not be able to comply.
If you move the start of the sync pulse left or right, you will move the screen left or right (same for the vertical pulse). The width of the pulse doesn't matter all that much, but don't make it too small, just move it by the same amount to retain its size, initially. If you change the overall size of the screen, by changing the 2200 or 1125, you will scale the screen larger or smaller. But when you do that, you will need to recalculate the pixel clock again.
So you can see from here there is no 1 single modeline for a particular resolution. There are very many! Moreover, with those 2 variations I described, you can scale the screen down to rid yourself of overscan, then recentre it properly. You only need to be aware that sometimes, you will hit a minimum or maximum timing value (or combination of values!) that the screen cannot handle, or a pixel rate that either the screen or the video card cannot handle. Hopefully, you will not, but if you do, you can see from above that there are many other combinations of these numbers that will generate the same resolution, screen size and position, so you will likely have to tinker quite a bit to get the combination correct.
Start by changing the overall screen size (2200 and 1125) and recalculating the clock. Just to get an idea as to whether you can scale the screen small enough. If so, then you can work on recentring it.
