6600 is the max i would take this engine.
the rev limiter set from the factory is there for a purpose. it is to keep the valvetrain from getting messed up or broken. Yet, it can withstand higher than 6000 rpms easily for just the reason that people do tach em up.
You dont need mods to shift higher than redline, just a tuner to raise the rev limiter. The mods will push/extend the powerband further up the rpm range, and the peak torque should stay close to the same.
If you have a dyno sheet, look at where you peak at most hp, that is the best place to shift. after that the HP table starts to fall and you will make less power, thus decreasing rate of acceleration, but if it is still more productive than the next gear, go for it.
it is all about track time and messing around with it and see where you get the best trap speed.
I currently have the rev limiter at 5500 with my sct tuner. its to keep my brother from taching the thing up time he drives it. plus, i dont street race and 5500 is enough for street purposes. At the track i will push it to a 6600limit.