You can of course push a turbo with more fueling to achieve more power. But a 40 isn't the best sizing for a 8.3 out of the box. It has a T4 foot. The problem you're going to run into trying to push the 40 is EGTs. Especially under load. Cummins specs the HX40 to 280hp, which is conservative. But real world you're looking at about 300hp tops before drive pressure and outlet temps are harming you instead of helping. (The function of a turbo is also it's downfall. It becomes a restriction on the exhaust side, and once it exceeds it's map the compressed air temp is so high, it negates any gain) Now something like a HX50 or HX55 is spec'd up to 370hp conservatively. This would get you closer to your goal.