I own a 98 Supercharged GSR, and will own a 5spd IS300. Frankly, I dunno why there would be a comparison.
Granted the stock GSR will win and lose some races (driver skill is a major factor here) to an IS300, why compare the both? Because they have similar acceleration times? People hardly compare the GSR with a v6 accord or a Maxima, the cars are meant for different groups and are in a different class.
Now, I think my GSR is funner to drive than the IS300 (and that's because it's a 5spd) but I'm sure when I get my 5spd IS300, it'll be just as fun if not more.
The IS300 has a better interior, quieter cabin noise, better ride, better leather, better brakes, firmer body and is a rwd. It costs more too. Unless you are so insecure about your IS300, why even bother to pay attention to the next person in his/her GSR?
When I'm in my GSR, I don't even look at zx2 or Neons. Sure some of 'em can run with me (the same way I can run with an IS300) but why bother? they're in a different league and probably could not afford a GSR