That else is done to the engine? If it is just I/H/C/E then it is not needed. If you are going to do turbo or rasing compression in an NA engine then by all means do one. You will not see big gains but it will improve high RPM breathing.
GSR do not have a valve job. Idk about the ITR. I know the ITR has a factory PnP but as for a valve job, idk. I am not to know too much about this subject. Here is a person who does, click me.
my old head was pitted in most valves, and i was losing compression cause of a burnt valve. i had a 3 angle valve job (regular valve job), milled the head and had them glass bead the head (cleans the shit out of it). all that mixed with a B18B manifold and cams helped out a lot. heres the dyno plot. notice how the powerband shifted 500 rpm up?