Porting a head, as long as its done correctly, will always be beneficial, regardless of whether the motor is Forced Induction or Naturally Aspirated.

I'm not huge on B series, but from what I've read, the only differences between the ITR head and GSR, is the ITR has slightly larger combustion chambers(I could be mistaken though).

So either way it will benefit the motor, but look into the actual differences between the 2 heads, and decide whether or not switching them is worth the trouble.