Shifting into 4th in your GT is like shifting into 3rd in an '05-'10 GT. Both are 1.32:1 ratios. So don't fret about having to do it in the 1/4.
WRT the rev limiter, the ECU starts closing the throttle plates approx. 300-400 rpm before redline to prevent the plates from slamming shut whenever rev limiter acrivates, which in turn reduces drivetrain shock.
Installing a tune will allow the rev limit to be raised as you had mentioned. But the limit may have to be increased a lot to prevent the 3-4 shift before crossing the finish line. And that's not good for the engine life if you do a lot of drag racing.
Suggest shifting at approx. 6800 rpm to avoid hitting the rev limiter, and see if that reduces your e.t., and increases your trap speed. Just remember the DA(ambient temperature, humidity, track altitude), track condition, and your launch prowess have a huge impact on your e.t. and speed.