Well if you're comparing car-to-car, bone-stock ITRs put down about 165 to the wheels. A typical i/h/e GSR will put down about 155 to the wheels.

As far as what's different from the ITR to the GSR, there's the throttle, intake manifold, head, cams, valvesprings, pistons, header and exhaust. So to "make" your GSR motor an ITR motor you would have to change all those things.

Of course if you're going to be spending money you might as well get the best possible parts than simply trying to match the ITR motor.