It will perform ~15% better on mining overall. ASICS perform ~500% better than GPUs overall. GPUs have long since become irrelevant in the bitcoin mining world. Even if major manufacturers were to add a specific SHA256 function to GPUs, the fact that the only thing ASICS do is SHA256 means that GPUs will never be relevant in the bitcoin mining world again. GPUs are, afterall, basically designed for graphics...
GPU is still meaningful as there are new models which will be out like SHA-3. No one currently do something with it. Only SHA-2.
If the ScryptASIC succeeds, then the game will be changed.