Good first try, and a good exercise to go through the math, but as has been pointed out:
1. Power/"dyno graph" in each gear will be different, especially in this car with the gear specific OEM (and aftermarket) mapping
2. Doesn't take into account the rpm/power you enter the next gear at. By your calcs you should shift at 6k in 5th. Look at the low RPM that puts you in for 6th. Think of it as "power under the curve". You want to maximize both the exit gear shift rpm power, AND the RPM/Power that you enter the next gear at.
O-cha has it, acceleration (G-force) graphs for each gear are the best real world data. Maximize the acceleration forces across your shifts. i.e. you want to maximize the combined acceleration from both the gear you're leaving, and the gear you're entering. Could also be done with dyno graphs, but you'd need them for each gear, and you'd be optimizing for best overall power from the gear you're shifting out of, and the gear you're shifting into.
Pure Stage 2/Vorsteiner/Supersprint/Helix V2/KW V3/Brembo/H&R Sway/Volk RE-30 + Sport Cups/Procede + Meth/AR Design + OEM Oil Coolers/M3 CF Trim/LED Steering Wheel/RIX Guage/Blacklines/GC Hybrid Camber Plates/Spec 3+ Clutch