You'll just have to see what it does on the day mate, what boost your standard T3 will hold depends on how well your engine flows (it's Volumetric Efficiency), if you've a modified head then you will flow better and the boost you can hold will be lower than if it was a un-modded head.
This does not mean you will make less BHP it just means it's easier for the gasses to get in and out of your engine and so backpressure/resistance (boost) will be less