Shift @ 2500 so you wont take any risk :-)
Cheers

 

Werd

 

Listen to Wave, as for the other advice...

...to reiterate what Wave said, shift where the wheel torque curves for the gears cross, or at redline if they don't. You would need to dyno your car in each gear (preferably on a Mustang to get the in-gear acceleration curves correct) to really get this right since transients make the torque curves different in each gear.

These guys explain it better than I do, with pretty graphs and all. They are talking bikes but the physics are the same:

http://www.sportrider.com/tech/146_0402_art/index.html

 

My graph starts falling off at about 6800 so I shift at redline in all gears and hold 4th as long as I need to to get through the traps, usually just over 7500. Other than that I don't go in the red as I see massive taper and the stock turbo is just out of breathe.