I can't speculate since I've spent exactly zero minutes in an N55, but I sure hope it pulls as hard and feels as strong as an N54. I have an LCI 335 E92 m-sport coming in early June and once I pass break-in period, whatever it may be, I'll be able to do a direct comparison with my sisters E92 335i '08. Both have step and are almost identical in options and equipment.
I've heard so much speculation regarding the move to twin-scroll, and I feel like butt-dyno proves next to nothing. The only definitive info we've got is that one guy's dyno chart and if someone runs 1/4s to get trap speed and time.
Here's my baseless opinion on why the move to single turbo.
N54 is arguably a more complex turbo setup with more parts and in the time since they developed the engine until now, they had an opportunity to develop a new engine with better fuel efficiency and a simpler turbo setup that theoretically might be more reliable down the road. However, the variable valvetronic on the N55 might ruin that argument since that has a lot more going on now.
As to why they used the N54 for the 335is, my guess is that they probably started developing that engine before the N55 came along. BMW might have introduced the N54 in 2007 and known all along that they were going to have to introduce a "zhp" type car down the road and started working on this earlier. Who knows....all speculation on my part.