saphires so far are a negative for unlocks as are asus - msi-his and gigabyte
http://www.overclock.net/t/1443242/the-r9-290-290x-unlock-thread
Unofficial 290 -> 290X Flash Results (Click to hide)
This will be updated whenever I find someone who flashes their card.
Some of these links have official benchmarks comparing them, but most don't. Some of these are also copies from the spreadsheet above, but I'm putting them in here just in case.
Sapphire
Unlocked: 0
Locked: One, Two, Three, Four Five, Six, Seven, Eight, Nine, Ten, Eleven, Twelve, Thirteen , Fourteen, Fifteen
Gigabyte
Unlocked: 0
Locked: One, Two, Three, Four, Five, Six
Powercolor
Unlocked: One, Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten
Locked: 0
MSI
Unlocked: 0
Locked: One, Two, Three, Four, Five , Six
HIS
Unlocked: 0
Locked: One
XFX
Unlocked: One, Two, Three, Four, Five, Six
Locked: One, Two, Three, Four (Some of these may be suspect- GPU-Z Bug incorrectly showing correct shaders)
ASUS:
Unlocked: 0
Locked: One, Two
Overclocking your 290 from 947Mhz to 1000Mhz grants you a ~3% performance increase. Successfully flashing your bios to a 290X will give you a ~4% performance increase (this is when both the 290 and 290x's clocks are the same). Combine it, and you get a ~7% performance increase after successfully flashing a stock 290 with a 290X bios.