The systems I/O limit you. Your iMac only has SATA 2 I/O (3 Gbit/s). The '11 iMac's with EFI update 1.6 applied will give you SATA 3 (6 Gbit/s).
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The systems I/O limit you. Your iMac only has SATA II I/O (3.0 Gbit/s). The '11 iMac's with EFI update 1.6 applied will give you SATA III (6.0 Gbit/s).