Hi Tim,
I dont know how to test the performance of the mmscd driver, but there was a lot of discussion about that module here on ZUG, that influeced the decision. So you will surely find some benchmarks here. One of the cases that could be makred as representative was a guy ( at german community) who couldnt resume anymore having opie, because he had moved some directories on a quite slow sd. After changing the driver, opie was there after resume. Search about the module here on ZUG, I heard some legend stories about, that it was postet by anonym, but I had no time to go in
The other point you are asking is about the kernel. If you take Joshs Kernel, you can get async access to your cards. The async option is disabled in the hentges Kernels. Joshs Kernel is somehow for very advanced users, who have moved directories to SD (or CF)because of the 64/0 segmentation.
You also should know what asynchron access means and how to control it by editing your fstab.
Cheers,
Sam