HDRI option is simply too slow for hi-resolution work

Yes and no, it all depends on the scene. While overall using HDRI slows down a scene because of more complex lighting and especially the soft shadows, I have used it in some large renders (4k resolution), and still been able to render out in under 30 minutes. (but, it is a simpler scene). Complex meshes or lots of pretty glass spheres will make render times shoot way up.

I find just about each scene is different and I have to make decisions on how "cool" I can make the render look without having to wait more than overnight for a finished render.

HDRI might work, but if that is too slow, I may use an faked HDRI workaround, (so I won't have to use soft shadows and then render faster).

Ambient Occlusion would be a great tool to have in Bryce. But, it would just be another tool, and like any other tool, not something to be used every single time, but as an option.

I can see Bryce 7 eventually having ambient occlusion, most likley through a plug-in.

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