I have spoken with the Team at Tuehouse and have posted the reply below. I hope this helps. Boost spiking is very normal on most turbo cars and the XR6T is no exception. Its not so much spiking but the boost dropping so at the top end caused by a flow restriction . A simply way to look at this is that flow = horse power, so 15 psi boost at 3000 rpm will be like 7.5 psi boost at 6000 rpm. Approximately. Double the rpm halve the boost, same air flow. So if there is a restriction in the system that will only allow X amount of air flow the boost pressure will drop accordingly Now this is caused by many parameters and included, exhaust system, intake system and turbo charger flow capacity. It can be cause also by the waste gate valve being forced open by backpressure in the exhaust manifold. This is why people fit a larger “pound” waste gate actuator, as mechanically this helps hold the waste gate shut. However it is best simply to lower the backpressure. So injectors were never going to help this problem, a new cat will help but will not eliminate it if you are looking for 15 psi across the board. It is important to know what other parts are fitted, If the intercooler and piping are poor this will also cause the problem. Eventually no matter what you do you will come up against the turbocharger itself simply not being able to flow enough air and the same will happen.