The only time I've seen EWG N55 dynos that have this sort of powerband shape are those N55's run in really hot/heat soaking temps and/or some issue that is cutting timing. Power should and climb and peak at ~6,000-6,200 rpm on EWG N55. You're right too, power should be in the ~340whp range. There's something going on that's killing power from 5,300-6,500rpms. Everything else looks pretty normal.
I see that the shop corrected to SAE. Most tuners correct to STD for power bragging rights. STD will get you ~5% higher numbers so corrected to STD you're looking at ~330whp/365wtq if that makes you feel better
As for comparing N54 vs N55 boost numbers, remember, the N54 uses tiny turbos vs the much bigger twin scroll on the N55. Point being, psi numbers between the N54 and N55 cannot be compared. It's apples to oranges.
Was your N54 car dynoed at the same shop? What did they correct the numbers too? STD or SAE? It all makes a big difference.
Most every Dynojet I've seen for the M2 shows 330whp and 350wtq in stock form. That's pretty consistent across the board.