Are you sure that you are using the 6Gb/s ports?
There are only 2 6Gb/s ports on the P67 and so you cant have all 4 SSD's working at full speed simultaneously.

Also, the C300's are running without the WriteBackCache enabled, so they are not really comparable.

Both are great drives though

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Will be posting some benchmarks on the Agility 3 60GB later today...