Well, it would be hard to maintain such list. In some older firmwares I had kernel with maximum being selectable via file in /sys/devices/platform/... which is IMO better way to solve it but lost the code and so far was lazy to write it again. But there was problem with it that it needed reinsertion of the card, which is hard to do when you boot from it :-) Current linux MMC code determines speed at card insertion and it is not prepared to change it later so it would need more effort than just changing one variable in MMC slot structure. But still it should be possible, I'll try to do it when doing something with mmc code again. I have some work in progress there (mainly SDHC compatibility for hacked SD cards) that waits for me to actually open the device and hack such card in and verify how it works. Part of the changes is upgrade to newer MMC code used in 2.6.18 that could maybe solve some issues with hanging whole kernel while card is detected (i.e. the issue OP has) and better error recovery in multiblock writes. Unfortunately there is lot of interesting stuff to do and so little of spare time.