I agree with most of what you say, except for the possible usefulness of 30 bit color. 24 bit is an aweful lot of color, and there aren't many video cards that support such a high degree of color. Considering how many Atari programs say "True Color not supported" and don't use more than a mono display anyways, I don't know if the "super color" is a good niche. It might be great for an Amiga clone as they tend to specialize in the video stuff anyways. I hope the final version has the Matrox on it to be honest, and that they aren't doing the Matrox driver just because the devel board uses it. Matrox has great 2D and video overlay/scaling support and very complete driver documentation. The 3D isn't the greatest, but it works. If you want the fastest 3D games, get an Xbox or something.
As for 8 bit alpha being a limited niche, alpha channel compositing isn't limited to the professional market anymore. The entire OS X display system uses compositing pretty heavily. Most of the web sites I design use 8 bit alpha channels, although, I will have to admit that the hardware doesn't have to support the alpha blending. It is nice if it does though. Alpha channel layering makes transparency and anti-aliased edges and such really easy. I gotta admit though that the Atari doesn't really have much support for alpha compositing either.
As for the 56K, I don't think anything you mentioned there is something that would be worth offloading to the DSP. Who can't decompress a JPEG so fast that transfering the decompressor and all the data back and forth to the DSP would take more time than using the CPU?
