2.6L 2835 combo is quite laggy but could be made to come onto boost earlier depending upon cam choice and tuning. With the RB30, they would spool up substantially quicker and would be a great combination. However, when switching from low mounts to high mounts the cost goes up considerably. The GTRS's and the 2835's are rated similarly, and therefore should make similar power.
I would stick with the GTRS's in a track car and enjoy the ride.
Also, there is little drivetrain loss difference between rwhp and awhp in GTR's due to the ECU-controlled torque split of the ATTESA system.