By modifying ROMs, McArti0 was able to obtain news resolutions on Amstrad CPC : 256x256 and maybe 1024x512 in an emulator like sugarbox.

He also wrote another thread on CPCWiki : CPC joining to VGA monitor by 5 wire interface which gives 512x464 in mode 2, complete with schematic and pinout.

In fact, you can reconfigure the CRTC chip in an Amstrad CPC and OS to use all 65 Kb of the video RAM. Amstrad can show a maximum resolution of 1024x512 at 25Hz (or 1072x480).