I would not say that forged is overkill BUT cast can be used well over 400whp when the tune is SPOT ON! Now your mileage may vary depending of the quality of the cast pistons. I doubt many are made to the quality of the original Nissan units. I have done 330+whp and 380+wtq on cast pistons. They did melt down durring the tuning on the dyno though. I can not tell you if it was the pistons fault or the injectors because the 440cc injectors went lean at peak tq. It was just too much for those injectors. Also the pistons were not original. I dont know what they were but they were an over bore.
In short. If you spend the money saved on someone with experiance tuning these cars then you should be foine with a cast piston to 400whp. I will tell you what sucks though. Spending $1,000 on forged pistons and blowing up the motor anyway because the tune is crap. Bottom line a crap tune with kill the motor either way you go.
Hope that helps.