G'day Eneal,
What drives are you running with the Adaptec?
We've had a lot of problems with Seagate ES.2's until you update the firmware from SNx to ANx.
The Adaptec KB article is here:
Ask Adaptec
Strangely they have now qualified the KB to state that the SN05 firmware (the level shipped to us mid 2008) is ok for the 51245.

The other thing to check is it may be network related, if you download the logfiles from DSS and look in the test.log for the hdparm entires. This should give you an idea of what the raw speed of the Raid0 array is.

Rgds Ben