All those mods, and no fully programmable ECU - There is the problem.
Get a Power FC, and tune it
The GT-RS is an awesome upgrade for an RB25, and yes it technically is a direct replacement for the stock turbo, but common sense should tell you that when you replace a stock turbo with one thats rated at 420PS, you are going to need a suitable ECU to run the show, and the required supporting mods.