First I have to repeat, that i am not part of the development team.
I am not working for Individual Computers in any way.
HighGFX-support is a result of some discussions on a1k.org and direct mails with Jens and Oliver.
The configtool and the firmware is done by Oliver.
I am only responsible for HighGFX (and HD720).
For 800x600 use Super72 modes.
(HighGFX and Super72 are very identical in operation)
You can use up to 64 colors for "useable" speed, with more than 6 bitplanes you will stress the chipmem DMA.
btw. all other modes with SuperHires resolution (Super72, dblpal, ...) are slow with 128 or 256 colors.
And they all have problems with FBlit and 128 + 256 colors.
(Allready noticed in the fblit docs ...)