I think it will have more of a benefit based on how much boost you run, moreso then the size of the turbo. So this would be more of a high boost mod. But everyone could see good to dramatic gains.
Larger turbo's would benefit much more than the stock/smaller ones. More airflow @ any given "psi" would make a heck of a difference if they were less restricted (through this mani instead of stock/626 ones).
I wouldnt' say it would be exponentially better, but larger turbos would definately benefit, probably following a 2nd order D.E. curve.