I can't test unless I have a board and it's a lot of labor for a few users.
I understand, maybe I can bribe you with some hardware when the time comes 
As for the number of users, that's in the lap of the Gods.
SDIO provides super performance but you need large transfers with a lot of buffer.
External RAM is supported, so buffer space should not be a problem.
I would be interested to know what Arduino applications would require more than a few MB/sec of I/O to an SD card.
That's probably the $64 question. I'm hoping to also have I2S support so maybe audio apps that analyse sound, a DSO that saves the waveform, logic analyser.
I guess I'm working on the "if you build it they will come" model, but they may not.
Rob