Well it seems that n54 seems to be best shifted between 6k-6.5 based off of dyno charts, after that the power drops off due to smaller turbos. Under casual driving, i personally shift at about 3k rpm because that feels right on my n52, on my fathers 335 i shift at about 2700-3000 as well. the turbo'd versions can be lugged more then the non turbos.
oops just realized you have an n55. I would say peak for n55 is 5500-6k but i have less knowledge on it compared to n54's.
3 stage manifold, Mapleridge headers, AA tune, Injen intake