The usb mass storage functionality currently runs only at USB 1.1 speed. Faster MMC speed doesn't help with this. You can try scp via wi-fi (card is in /media/mmc1 on the device). For windows there is easy client called WinSCP. Needs openssh or dropbear installed on the device. Wi-fi has its own limits too (may depend on encryption level) but it should be faster than USB 1.1.

Faster MMC is currently helpful when booting from the card, for the swapfile and for aplications that have lot of data on card.

As for usb 2.0 I don't know. Nokia marketing says it supports USB 2.0 for client mode http://europe.nokia.com/A4145105 but this can be mistake. I hope it is not. Currently USB 2.0 functionality in g_file_storage kernel module is not enabled. Maybe there is a reason.

As for the 4x speedup, this is optional feature and some cards may not support it. I just tried quick hack and hardcoded it and it seems to work with Kingston 2GB card but it needs to be checked properly and enabled only for cards that support it.