you must remember amiga rtg modes do not neccesarily fit vga modes as you can re-design them via p96 prefs as example, pixel clocks, timings, dimensions.. . aros is more limited in this respect at least for now, however i think overscan functionality is practically only neccessary for pal and ntsc type displays.
what concerns text and gfx overscans i think it was depending on a program which values it was asking up. text editor would use text overscan, while painting package or some game, or a animation player would go for gfx overscan.
correct me if im wrong.