-i 5 is max for 280x, but I have modified timings on my sapphire with elpida memory at it doing 330h/s now, but it's sometime unstable because of huge undervolte. But since Claymore v12.4 have great whatchdog, I don't care about unstable work of cards, miner just restart cards in case of crash.
If u don't care about electricity, I think sapphire 280x could have even better results after card acceleration.

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Mine is Elpida memory too, electricity is cheap where i live. can you explain how you did all of that or give a link if possible please?