The only turbo experience that I have aside from my 335 has been with Garrett turbos. I ran a GT30R on my s2k and I LOVED that turbo for that car. All of my friends with various cars (several high HP GTI's, Evo's, turbo s2k's) all ran Garrett GT series turbos without any problems. Even when they ran GT35R's or GT40's on a small 2 liter engine, they would spool up pretty quickly. They are water/oil cooled ball-bearing turbos that have a pretty good reputation, and they make different sized turbos just for about every application. Now the question that comes into play is how much room is there in the engine bay, and how big can you actually go if you stay with twins? Twin GT28R's sound nice, but would they fit? Also, larger turbos mean larger wastegates that require more room. Maybe converting to a larger single turbo would be better since it should still spool quickly with the relatively high compression and the decent sized displacement of the N54. Another thing is that you will need new manifolds since the stock ones are a one piece design with the turbine. Also, once you have the new turbo(s) to provide you with the boost you want, what will you do to supply them with more fuel? Its not like you can pull some RC injectors off the shelf.
Seems like there will be a lot of work ahead of you, but whatever route you decide to go I wish you the best of luck with building up that beast!
Keep us posted.