From PHR Website:
If your IS300 is a daily driver and must be reliable on a constant basis, we highly recommend staying at a level equivalent to Stage 1 (300 RWHP). The 2JZGE engine used in the IS300, while similar to the version used in the Supra, is not built as strong as the Supra version and cannot tolerate high boost levels reliably. It is commonly agreed that 480-500 RWHP is the maximum safe limit for the IS300 GE engine, although there are examples of higher HP limits for short periods of time. Keep in mind that all warranties on the car will be voided with the installation of an aftermarket turbocharger system, so be sure to get the facts on reliability! For automatic IS owners, keep in mind that the factory automatic transmission begins to randomly fail at levels above 350 RWHP. Some owners will never experience a problem, while others will not last a month. By comparison, the manual transmission version has survived testing well at power levels exceeding 400 RWHP, with the inclusion of a high performance clutch upgrade. At Powerhouse Racing, we own and drive automatic and manual transmission equipped IS300’s on a daily basis. We have tested them under various conditions, and pushed them to the limits to know when and where they fail so that the end customer can hopefully avoid that experience.