4096: (OC 550)
There's no caching on write aparently (probably to avoid lost data).
So writes must be mostly dependent on class + SD reader/writer driver.
Acording to wikipedia:
Class 2, 2 MB/s, slowest for SDHC cards.
Class 4, 4 MB/s.
Class 6, 6 MB/s.
Class 10, 10 MB/s.
(this is write speed, MegaBYTE).
So... either we are getting bogus results, or our cards are a little above the quoted class.
Read 1st post. Follow Link. Read whole 1st post there. There's a sample script there.
You must create a file in /system/etc/init.d/ named 99something_you_chose (should begin with 99, the files are executed in order, we should have our script run last).
So... either you remount /system with read+write permissions, and put the file there, or edit the androidinstall.tar and put the file there, or a androidupdate.tar with the file there.
Been playing with this config only a few days with wifi only. My understanding is that the ext4 file system speed is what makes the system very usable. The only thing I find frustrating is the time it takes to boot and become stable. I say try it on your current sd first. The boost in reading seems to help starting apps and using file managers.