IHI doesn't publicly publish their compressor maps. That being said fuel is the main contributor to more boost. Higher octane/E85 or meth injection is really the only way to get more without turbo modification. Or get the turbine wheel clipped or change the turbine housing to a P20 but injectors will be your limitation. If you are looking for more power try TGV deletes,ported/polished exhaust manifolds or a lightened crank pulley.