>  Its a limitation, nobody wrote code to handle > 8bit for Cirrus.

  Actually, we tried, years ago. One big problem was big- to little-endian 
byte swap, which rendered dog-slow on the Amiga.

  I had some sort of 16- and 24-Bit "pnmtoCirrus"-display tool, which 
directly poked into the frame buffer. I lost the source over the years, 
though. Required kernel hacks as well, to implement some io-controls 
(display on and off and such).

  BTW, X11 used to be 8Bit-only for decades..

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Markus Illenseer