Figure an ITR makes 20 hp more and it has a bunch of mods over the GSR including higher compression pistons, a mild port job on the head, a different intake manifold, slightly larger throttle, better flowing exhaust, etc. Granted these are pretty incremental differences but the gains from just the cams will obviously be less than the total 20 hp. I'd say 8-10 hp from just bolting in the cams and doing nothing else.

The way to maximize your gains when upgrading cams is to get adjustable gears to let you dial in the cam timing, as well as some kind of electronic means of tuning beyond a plain chipped ECU. I would say a V-AFC at the minimum. Both the gears and whatever electronic tuning device you use need to be tuned on a dyno in order to produce any gains.