The S54 is 3.2l, which is 0.2L more than the N52, that could explain some of the weight difference.
In reality, the 2JZ is also an iron block, yet I'm still well below a regular Z4M weight. As I mentioned, probably more to do with all the other lightweight aftermarket updates I made..
Wikepedia states the S65 weighs 30lbs less than the "replacement engine", or the S54. I don't think you'll see that much of a drop when all is said and done.