I did each transfer twice to reduce the likelihood of running into some bizarre exception. Based on my speeds, I guess I have a faster SD card in my Wii than most. Still, the increase in speed is dramatic.
Sin & Punishment – 287 blocks
Wii -> SD: BEFORE – 2:43, 2:45
AFTER – 0:32, 0:32 (80% faster!)
SD -> Wii: BEFORE – 1:08, 1:08
AFTER – 0:40, 0:40 (41% faster)
Super Mario Bros 3 – 27 blocks
Wii -> SD: BEFORE – 0:47, 0:46
AFTER – 0:04, 0:04 (91% faster!)
SD -> Wii: BEFORE – 0:10, 0:10
AFTER – 0:07, 0:07 (30% faster)
Mega Man II – 17 blocks
Wii -> SD: BEFORE – 0:15, 0:14
AFTER – 0:04, 0:04 (72% faster!)
SD -> Wii: 0:08, 0:08
AFTER – 0:06, 0:06 (25% faster)
I imagine that in general, the smaller the file, the less dramatic the improvement due to initialization and clean-up steps, which are constants.