Aren't you forgetting that Garrett make the HKS turbos.
From my experience I have found the HKS equivalent to be a laggier turbo but when the boost rises and higher flow is required the HKS turbo will now be in the meat of its efficiency curve while the standard Garrett equivalent will be past its optimum and starting to become inefficient. The garrett will keep producing boost but with a higher shaft speed and higher temperatures.
The use of the names 2530 etc by HKS is more an indication of external size and flanges than internal parts. Knowing how the Japanese love 'plug & play' or 'bolt-on' options we can see why HKS would use the names of Garrett turbos that can be directly replaced by their own.