Well some people say that the ITR/b16's heads flow better than the GSR's, but the GSR has a wedged combustion chamber raising compression. ITR head on GSR block is really the same as an ITR motor almost, it is considered a poor mans' itr motor, with a little less compression. You should be running 9.8:1 compression with the ITR head on the stock gsr block, or roughly around there.
Now back to the flow characteristics of the GSR vs. ITR/b16 head. I believe TOO did a flow comparison of the two. I think the GSR outperformed the ITR down low and the ITR flowed a little more up top over the GSR or it might be the other way around, i can't seem to remember for sure. Also the ITR head flows a little more than the stock b16a head since it is slightly polished up from the factory.
Milan