I currently have nearly the exact same problem. I need to eliminate the overscan on a 720p HDTV.
Is one of those apps, SwitchResX and DisplayConfigX, better than the other? Can either display a test resolution of 1248x704 before buying it? DisplayConfigX doesn't seem to be able to show it to me first. I'd like to know it'll actually work before I pay for it...
Is there really no plist or something anywhere that one could simply add a custom resoltution to?
FWIW, the 1:1 mode (PC-WIDE) on my Samsung leaves black borders on all four sides. It did the same thing in Windows (old HTPC, Mini is my new HTPC), although I could use the regular WIDE TV mode and set a custom resolution in the ATI control panel to compensate for the overscan. That looked great, you'd never know it was slightly scaled and my menu bar actually showed. Most of the time I VNC/Screen Share into the mini anyway, but I still don't like the overscan.