It is obviously not a limitation of the 1438, as I just got an RGB-VGA adaptor which I used to connect the A1200 to my Trinitron monitor. The image looked the same as on the 1438, with the black bars.

The VGA monitor does have the option to stretch the image horizontally. I'm not sure whether it was meant to be stretched though. It didn't look any worse when it was stretched.

The only problem with adjusting the picture to suit the Amiga is that when I switch back to the PC, the BIOS and Windows 2000 boo screens are the wrong size and in the wrong place, so I'm guessing they are the same resolution/scan rate as Multiscan productivity.