I don't know if it's reliable or not, but I did 3 tests on my DHD, AR 6.3.1 mod, quite a few apps installed, half-full SD card a KINGMAX 16GB Class 6 which i bought 2 days ago. My previous card was a SAMSUNG 8GB Class 2 (the one that came with the phone).
What's interesting to note is that, after doing the first test I thought to myself "hey, why not overclock the cpu from 1.1GHz to 1.6GHz?" and so the second test went better than expected. Then I went from 1.6GHz to 1.2GHz and I got slightly better results than the 1.1GHz. See for yourself:
1.1:
1.6:
1.2:
Coincidence much? Or working as intended?
Notes: Both cards are SDHC. OC has been done with Daemon Controller. 1.1=1152000. 1.2=122880. 1.6=1612800.