I agree with the 303mm barrel. If you're looking for one, I have a PDI 6.01 303mm that hasn't been used very much at all that I'm looking to sell. PM if interested.You are getting a lighter rifle, but you still are not going to get 303mm barrel? If I were you, I would get the same as you, but a 303mm barrel, cut the m150 spring so piston won't hit cylinder head, making the gun dead silent, and I would still hit ~400 fps with 0.36, and would reach nice 200ft easily.
But if you want that longer barrel, get tokyo marui precision chamber, it comes with a 430mm tbb.
Other than that, your build looks good. I'd maybe go with the TM Precision Chamber and the Nineball bucking, or just stick with the stock chamber with mods and then get the Precision Chamber later on.