SKTools has some tests in its "Benchmark" category that would be good for this. These in particular:
- Storage Card (write)
- Storage Card (read)
- FL: Storage Card
I've found the results can vary greatly with different ROMs and of course it has a direct impact on any programs that use the SD card storage. I've got a 16GB class 6 card.
This hack helps speed things up in some cases, but there are some ROMs that hack has no impact on at all.