Yes, you can copy it, my car will evolve anyway to be the greatest skyline-stereo in the world anyway.
Yes, you can substitute the subs, they will generate enough bass anyway. Or why not this? Rockford Fosgate PL2210
About the power, it sounds really good in my car, I can promise you I feel it is enough. Sometimes it feels like the door to the trunk is going to pop.
Regards
Håkan