No, a VLIW4 SP is most likely a tad slower than a VLIW5 SP. But a 1536 (384) shader VLIW4 chip will have 20% more real useful SPs than a 1600 (320) VLIW5 chip.
A 320x5=1600 chip will have a lot less useful power than a 384x4=1536 chip.
Even if a Cayman shader is 96% of a Cypress one performance wise, there are still 20-50% more of them depending on which rumor to trust.
Your number of 20% higher performance is based on 20% more shaders.
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