test drove N55 on 2011 335xi E92 last Saturday, I think I am agreeing with C/D on this one.
I have 2008 535i with sport auto GA6HP19Z, with faster shift time than regular auto I test drove; not to mention the 335xi is AWD but the weight is about the same between the two.
First the exhaust sound is superb, throaty but not obnoxiously loud. The N55 seems noisier maybe the missing damper material in the engine hood contributes to that; but I didn't hear the turbo.
The acceleration is comparable to my 535i, but the N54 is super smooth and gives you the feeling effortless and the N55 seems raw and throw you into the back seat harder from the hole and rev happy but then it feels plateau-ing sooner in non DS mode.
Being an amateur it's hard for me to isolate just the engine different so I'll say the two is very close but N54 definitely feels a bit stronger especially at the top end. Again itís hard for me to compare broken in engine to brand new engine (so take this comment with grain of salt, alot of it), also I really tried to keep the test car away from redline, itís not cool since itíll be someoneís car someday. All in all I am looking forward to my own N55 335xi in June, maybe after an extended time behind the wheels I can point out the diff netween N54 vs N55 better.