A number of guys have done this on the 07-12 GT500s as well. There seems to be no advantage to putting a larger than stock S/C on in conjunction with the turbos so most leave that part completely alone (not even a smaller pulley).
The advantage the compound boost guys tout is the low end response of the S/C with the top end torque of the turbos.
Seems though, removing the S/C and only running the turbos is the way to go to produce the largest numbers. Low end grunt generally not being your biggest problem in the GT500