Couple issues with above:
1. Your calculations make some assumptions that aren't true. The main one being that N54s run anything more than about 10-11 psi at 6500 rpm. Boost ramp down after 6k is precipitous regardless of what it was prior to 6k. Figure 17 psi at 6k, but even that is pushing it. Most 17 psi power runs tend to make peak power at around 5500 rpm, if on 100+ octane and basic supporting mods = ~410 whp, give or take 10 whp.
2. Most running that amount of boost are going to aftermarket intercoolers that are looking at 1-1.5 psi of pressure drop at MOST at those flow values (~42 lb/min). That's hardly the ~5 psi you are using. Frankly, I don't agree with the nearly 3 psi pressure drop across the IC at stock boost pressures that you are figuring either, and neither does the pressure testing data that has been carried out on the stock IC vs. upgraded ones as far as before and after comparisons.
'13 Black Sapphire Metallic 335is DCT, BMS CP/DCI/DPs, MHD E40, Wagner Evo 2
'14 991 GT3, Sapphire Blue / Black, 11.30 @ 124
'07 997TT 6MT, Arctic Silver/Sea Blue: FBO E85, stock turbos: 5.9 60-130 (now with 656 whp, 700 wtq)