I'm sure that I've seen a comparison somewhere on the differences with the dc2 engine, but can't remember where... Might have just been a reply to a comment or something - I'm sure there'll be something online comparing the gsr engine to a jdm dc2 one though.
Off the top of my head though you're looking at pistons, crankshaft, better flowing head (I'm sure that the type r ones were hand finished), valve springs, and probably more. I'd have said a custom map as well to bake sure it's working best for your setup
Jake @KiNK43 is your man here though as he just built a beast of an engine even without the turbo.