Xorg can do even depth 24 on m68k.
It's deppend only of graphic card used on macs.
And surely of the settings used in macos.
And also of mac system.
The only chage to do in
xorg.conf is to comment this line
" # DefaultDepth xx "
The rest of settings _ display-mode, and depth_ are made in macos.
For q630 machine, passing from mac_sys_7.5.3 to 7.5.5, gave me
the choise of "800x640" display, in 8 depth.
I can confirme that fbdev in xorg 7.1.1 may do to 24 depth on
IIcx - 68030 16 MHz , less 4 MB VRAM, 20 MB DRAM
quadra700 68040 25 Mhz , native FPU present
( test a supermac graphic card, and RasterOps, both Nubus cards)
In general xorg/ fbdev do good jobs on 68030 , no issue on fpu.
tests on :
lc II 68030 16Mhz, less 1 MB VRAM, 6 MB DRAM
lc III 68030 32MHz,
Regards,
-ee