According to Zers in Japan who have tried it, the IHIs have better boost response compared to the GT2530s. According to them, the GT2530s goes somewhat flat above 5000rpm... (I know this is at least true on single GT2530 Silvias)
The IHIs are rated at 550rwhp (street) and 600rwhp (drag/short term), while the GT2530s are rated at 480rwhp (street) and 550rwhp (drag/short term). These are manufacturer specs...