First off:
Our stock block, rods, etc is VERY stout. Just to give you a reference, the stock rods on a 4g63 are toothpicks compared to ours. 400whp should be easy.
Secondly: FORGET ABOUT OVERBOOSTING! The ecu monitors the MAP (manifold absolute pressure) and will retard spark rev limit with an overboost condition until the MAP goes down to stock. Its actually in the manual, and its a constantly monitored code that does not trigger a CEL.
Overboosting with such an overworked turbo is a bad idea anyway! That turbo is really at its limit providing us the power it does now. Its most likely at the limit of its efficiency and overboosting it will only cause you to overheat the air being pushed through it, thus lowering hp by having a hotter intake charge. Now, its ok to use an MBC to get it to HOLD the same amount of boost for more RPMs, but, increasing the overall boost pressure is a bad bad idea.
Honestly, running the same amount of boost with the bigger, more efficient turbo will in itself increase hp by lowering intake charge temps. Same boost psi, same hp over a wider powerband and lower intake temps due to increased efficiency = increased power and reduced quarter mile times AND will be safer for the car in the long run!