with that out of the way, I can now speak to an FBO N54 vs FBO N55, same mods, same tune. My N54 was a 2008 6MT, and my N55 is a 2011 6MT.

everyone always talks about how much better the N54 platform is for power due to forged internals (truth) and the fact that the N55 has a single, smaller snail and can't flow as much air through the turbo (also truth). 

I'm finding that down low, they both feel the same when you aren't running meth/E85. Stage 2 fbo provides an incredible throttle response increase on both engines. Both N5x engines were designed to be low-midrange motors, and they definitely die up top. But it's been 8 months since I drove my N54 and I can safely say that it holds more boost longer by far than the N55 and you can definitely feel that up top. I can readily feel the N55 drop off and run out of breath on the stock turbo as early as 4500 rpm. I could feel the same type of wall in the N54 above 5500 rpm. The gas pedal is to the floor but the boost just tapers a LOT sooner on the stock N55. It cannot hold as much boost for as long a period of time. Peak target was still 19.4 PSI but by 5500 or so, I could hear the diverter valves releasing pressure significantly. 

In pure performance, my conclusion is that if you don't want to get serious about mods and are interested in just the basic FBO setup, then either engine is more or less on equal ground. N54 will almost surely dyno a bit higher ( I will confirm this when I am on another dynojet on the 18th ), but not so drastic that it wouldn't be a driver/traction race between the two. The reason to go N54 are really when you start talking meth/E85 because then you can start raising the boost, and the stock N54 turbos just support more boost, plain and simple. If the PURE Stage 2 N55 turbo pans out as not blowing up the bottom end at 500 whp, then I would argue in most cases (for most enthusiasts) it will in the future become a wash as to whether you get an N54 or N55, and unless you want 700 whp (which requires an N54 if you want to run stock internals), then I would recommend either engine. 

Subjectively, the N55 sounds a hell of a lot better than N54 in terms of engine/exhaust note when fully bolted on. by "better", i mean less raspy, much deeper tone. If you like the rasp, then N54 is for you. 

All things considered against the stock N55 snail, I'll definitely be looking at the PURE Stage 2 over the next year. I have no desire to maximize boost on an aftermarket turbo like that, however, I would feel better with a turbo running 21 PSI that I know isn't being strained at its efficiency limit. 425-450 whp would be sufficient. In the immediate I am probably going to pick up a bms meth kit to get more warm fuzzies about the higher boost levels I'm running, as well as help mitigate the heat soak issues of these engines.