I managed to find a few minutes and put together the promised patch series for the latest n800 firmware. This series adds support for MMC 4.x wide-bus support and restores 48MHz highspeed support. I did some basic testing with my n800 and confirmed that transfer speeds are now back to what they were with my older patched kernels.
As I said previously, Nokia made deliberate decisions to exclude both of these features from the official kernel, so you might perhaps approach my kernel with some caution, but the older iterations worked well for most people, including myself. As always, if you break your n800, you get to keep both pieces.
You can find the patches and the prebuilt kernel image here.