Having a look at the infamous utilisation chart (3 posts back), you get approximately 50% of the bitterness utilisation at 30 mins in comparison to the utilisation after 60. So, there is bittering, only half as much.
Saying that, I have made this recipe to the original specification, and I reckon a longer boil would not go astray. Make no mistake, there's a good deal of hoppiness in it, but imho, it could carry a few more IBU's. Maybe try extending the 30 min additions out to 45 mins, but keep the 20 and 10's as they are.
Your mileage may vary.
I am drinking a version of this recipe now (I used cascade & armarillo) and would agree re: longer boil would improve it. This was my first go at extract and learned a lot, plus as an added bonus I got some very drinkable beer!
Can't go back to kits now - just bottled Smurto's JSGA and the sample was very promising. Heatwave here in Brisbane caused some problems but I kept it below 24 at all times so hopefully it'll be a winner.