Hi, I think if you want to go for stage II, you can do it!! I`d heard you must modify your autotranny by changing more larger torque converter and hydrosystem valve(??). In addition, the auto tranny controlling chip should be also upgraded. However, I think there`s additional problem, which is your engine. I am not sure whether those higher boost will be okay. I mean you need to change piston to a forged one, if you wanna go more than 350-400HP. I even don`t know when forged piston is needed in terms of horsepower. 350-400HP is just my guess from my research.
By the way, I`d heard it`s only matter of chip, when you upgrade power of your auto tranny car. That is to say, if tranny chip is well modulated, I`d heard there will be no problem without modifying your actual part of auto tranny.
In my opinion, it`s wierd why auto tranny is not recommended for supercharger kits. Don`t you think there will be less risk as gear is automatically changed? NX express guy says auto tranny is the right one for NX kit. Then, why supercharger or turbocharger not?
Let me know anything you know about this issue, because myself also had been looking for the way to upgrade my autotranny for more than six months. I don`t want to have manual, and I just want to have 350HP or better torque than stock E46 M3 with less than 8K. My car is 2003 BMW 325xi auto tranny.