2nd on the 6 speed thing:

You drag racers never get the car out of 4th gear, so why not just convert the car to 4 speed?

On a road course, above 100mph, you have a lot more options with the 6 speed as compared to either having the car loaded up nice in 4th and being a dog when you go to 5th up the hill at Watkins Glen.

If you're just streeting the car with a little drag duty, get your 5 speed rebuilt by TRE.
If you're sick enough to do road course stuff, in order to convert to the 6 speed, you need:
The 6 speed gearbox, of course.
6 speed boot/shifter and new shifter cables
6 speed right side trans mount.

As for the t-case, the GSR works and you do not have to convert the car over to the ACD T-case.

The parts are not cheap, but if you need it for your application, I'd say it's worth it. If you're just doing it for street cred, just get the 6 speed shift knob. Your wallet will thank you.