Testing the Toshiba panel for the 540c in Grayscale mode, it can display 27 distinct shades (Black, White & 25 shades of gray inbetween). This means it should be capable of displaying 19683 colors @ 640x400. So, Apple's estimate of 24000 isn't too far off. Since it can clearly display more than 16 shades, it must be a 15-bit panel. Really quite good for that time period.
So, the 540 panel is indeed superior to the color panels in displaying grayscale, since it is capable true 64 grayscale. However, this makes the 540c superior to any of the 4-bit grayscale panels.
Unfortunately, the 540 panels suffer from terminal tunnel vision, as do all grayscale active matrix panels of that time period.
I'd be interested in someone with a Sharp 540c panel testing the grayscale gradients in grayscale mode. Since the colors on the Sharp panel appear more vibrant, I expect it is capable of displaying the full 32 shades of gray.
So, the 540 panel is indeed superior to the color panels in displaying grayscale, since it is capable true 64 grayscale. However, this makes the 540c superior to any of the 4-bit grayscale panels.
Unfortunately, the 540 panels suffer from terminal tunnel vision, as do all grayscale active matrix panels of that time period.
I'd be interested in someone with a Sharp 540c panel testing the grayscale gradients in grayscale mode. Since the colors on the Sharp panel appear more vibrant, I expect it is capable of displaying the full 32 shades of gray.