The engine will continue to make power if you rev it to 6500 rpm or higher. The problem is with the TCU. The rev's climb so fast you need a large gap between the ECU limiter and TCU shift points. If not you hit the ECU limiter. My ECU has the limiter set at 6500. My TCU WAS set at 6400 and I hit the ECU limiter every single time in every single gear. I went back to the stock shift points in the TCU and I still hit the limiter occasionally during the 2nd to 3rd shift. Perhaps because of changing the diff to a 3:27. In other words, to shift at 6400 you'll need the ECU set real high. Based on the way my car acted and unless someone is doing it better now, I'd say 7000. Since your getting a dyno tune from E/C you should talk to them about what's possible with the TCU. I've never used nitrous so I won't give my uneducated opinion. BTW, on a Mustang dyno, my car made 20 hp more at 6400 than 6100 with similar mods as yours. (stock injectors and Pierburg fuel pump)

Les